[TLDR] (the short version)
The particular configuration of partitions shown in OP makes it impossible not to have to rebuild / restore the bootloader after moving the head end of the /boot partitions about.

[The complete story]
What I did:

• boot on live-Linux with a bootable USB drive in UEFI mode. (I chose to use Ubuntu 18.04).
  Do mind the fact that Ubuntu and modern NVIDIA discrete graphics do not get along at all. If you have a recent NVIDIA graphics card (recent.. say as of ca. 2017), you will save time if you actually UEFI-boot a live OS without GUI. That means performing the repartitioning on cli and possibly the subsequent restoration of the bootloader too.

• with GPartEd reclaim partition 2 and MOVE partition 3 (swap) left, so it becomes contiguous with partition 1. Simply "moving" it does not change the partition's UUID. If you erase it and recreate it elsewhere (your call entirely) you will have to later vi into /etc/fstab to modify the corresponding swap's entry's new UUID.

• move /boot (partition 5) as far left as needed. Moving the head end (i.e. the left limit on OP's graphical representation) of that partition requires that you later restore the bootloader (GRUB2's in my case). Otherwise you will fail to boot. If unlike in the present case /boot and / (root) were on the same partition, moving the head end of / (root) would also require restoring the GRUB bootloader (because it would mean re-indexing the start of the /boot/efi information located in the root partition).

• move / (root), /var and /home at whim.

The above constitutes the full answer to my original question.

Restoring the booloader is out of scope here, but is well documented, now even for UEFI. Look up this and this for detailed help. Yr mileage may vary according to the distro you have, but my advice is: use generic cli instructions as much as you can. They are best documented as in chroot's case for instance, whereas wrappers (again as in chroot's wrappers) may not be. Again, your call.

HTH anyone with doubts similar to those I had.

[TLDR] (the short version)
The particular configuration of partitions shown in OP makes it impossible not to have to rebuild / restore the bootloader after moving the head end of the /boot partitions about.

[The complete story]
What I did:

• boot on live-Linux with a bootable USB drive in UEFI mode. (I chose to use Ubuntu 18.04).
  Do mind the fact that Ubuntu and modern NVIDIA discrete graphics do not get along at all. If you have a recent NVIDIA graphics card (recent.. say as of ca. 2017), you will save time if you actually UEFI-boot a live OS without GUI. That means performing the repartitioning on cli and possibly the subsequent restoration of the bootloader too.

• with GPartEd reclaim partition 2 and MOVE partition 3 (swap) left, so it becomes contiguous with partition 1. Simply "moving" it does not change the partition's UUID. If you erase it and recreate it elsewhere (your call entirely) you will have to later vi into /etc/fstab to modify the corresponding swap's entry's new UUID.

• move /boot (partition 5) as far left as needed. Moving the head end (i.e. the left limit on OP's graphical representation) of that partition requires that you later restore the bootloader (GRUB2's in my case). Otherwise you will fail to boot. If unlike in the present case /boot and / (root) were on the same partition, moving the head end of / (root) would also require restoring the GRUB bootloader (because it would mean re-indexing the start of the /boot/efi information located in the root partition).

• move / (root), /var and /home at whim.

The above constitutes the full answer to my original question.

Restoring the booloader is out of scope here, but is well documented, now even for UEFI. Look up this and this for detailed help. Your mileage may vary according to the distro you have, but my advice is: use generic cli instructions as much as you can. They are best documented as in chroot's case for instance, whereas wrappers (again as in chroot's wrappers) may not be. Again, your call.

HTH anyone with doubts similar to those I had.

[TLDR] (the short version)
The particular configuration of partitions shown in OP makes it impossible not to have to rebuild / restore the bootloader after moving the head end of the /boot partitions about.

[The complete story]
What I did:

• boot in Linux with a USB drive in UEFI mode. (I chose to use Ubuntu 18.04).
  Do mind the fact that Ubuntu and modern NVIDIA discrete graphics do not get along well at all. If you have a recent NVIDIA graphics card (recent.. say as of 2017), you will save time if you actually UEFI-boot a live OS without GUI. That means performing the repartitioning on cli and possibly too the subsequent restoration of the bootloader.

• with GPartEd reclaim partition 2 and MOVE partition 3 (swap) left, so it becomes contiguous of partition 1. In moving it, you keep the same UUID. If you erase it and recreate it elsewhere (your call entirely) you will have to later vi into /etc/fstab to modify the corresponding swap's entry's new UUID.

• move /boot (partition 5) as far left as needed. Moving the head end (i.e. the left limit) of that partition requires restoring the bootloader (GRUB2's in my case) to be able to boot later. If unlike in the present case /boot and / (root) were on the same partition, moving the head end of / (root) would require restoring the GRUB bootloader (because it would mean re-indexing the start of the /boot/efi information in it).

• move / (root), /var and /home at whim.

The above constitutes the full answer to my original question.

Restoring the booloader is out of scope here, but is well documented, even for UEFI now. Look up this and this too. Yr mileage may vary according to the distro you have, but my advice is to try to use generic cli instructions as much as you can. They are best documented as in chroot's case for instance, whereas wrappers (as in chroot's wrappers) may not be. Again, your call.

[TLDR] (the short version)
The particular configuration of partitions shown in OP makes it impossible not to have to rebuild / restore the bootloader after moving the head end of the /boot partitions about.

[The complete story]
What I did:

• boot on live-Linux with a bootable USB drive in UEFI mode. (I chose to use Ubuntu 18.04).
  Do mind the fact that Ubuntu and modern NVIDIA discrete graphics do not get along at all. If you have a recent NVIDIA graphics card (recent.. say as of ca. 2017), you will save time if you actually UEFI-boot a live OS without GUI. That means performing the repartitioning on cli and possibly the subsequent restoration of the bootloader too.

• with GPartEd reclaim partition 2 and MOVE partition 3 (swap) left, so it becomes contiguous with partition 1. Simply "moving" it does not change the partition's UUID. If you erase it and recreate it elsewhere (your call entirely) you will have to later vi into /etc/fstab to modify the corresponding swap's entry's new UUID.

• move /boot (partition 5) as far left as needed. Moving the head end (i.e. the left limit on OP's graphical representation) of that partition requires that you later restore the bootloader (GRUB2's in my case). Otherwise you will fail to boot. If unlike in the present case /boot and / (root) were on the same partition, moving the head end of / (root) would also require restoring the GRUB bootloader (because it would mean re-indexing the start of the /boot/efi information located in the root partition).

• move / (root), /var and /home at whim.

The above constitutes the full answer to my original question.

Restoring the booloader is out of scope here, but is well documented, now even for UEFI. Look up this and this for detailed help. Yr mileage may vary according to the distro you have, but my advice is: use generic cli instructions as much as you can. They are best documented as in chroot's case for instance, whereas wrappers (again as in chroot's wrappers) may not be. Again, your call.

HTH anyone with doubts similar to those I had.