Here is a slightly different take.

First of all: learning the the absolute positions of the note is not all that hard. On a 21 fret guitar there are only 126 different notes you can play and only 45 of these are unique. There are only 12 different notes in the halftone scale: you just spent 10 minutes only playing C# you quickly get the hang where all the C# are. Repeat with a different note tomorrow and after two weeks you have it down.

Secondly: In many cases the relative position of the notes is much more important and useful. The role and musical function of a note is not determined by its absolutely value but by it's value relative to what else is going on. An A in an A Major chord has a very different role then in an F major chord or a Bb maj 7 chord. (By the way A, F, Bmaj 7 is fun progression to play if you keep the A on top).

That's a lot what the Nashville numbering system is about. (https://en.wikipedia.org/wiki/Nashville_Number_System). Instead of writing Dm7, G7, C they would call it a II, V, I. The idea here is that a II, V, I progression always sounds the same regardless what actual key it is in. (Dm7, G7, C or Am7,D,G or Cm7,F7,Bb, etc). It's (IMO) more important to recognize relative jumps, intervals and progressions than absolute notes.

That's where the guitar comes in handy. The intervals and scales correspond to certain shapes, so you need to learn those. Again, doing 15 minutes a day on a certain scale will very quickly get you there. Same with chord shapes. Guitar makes transposing also relatively easy: you just shift the riff, scale or chord shape up and down the neck (within reason).

Once you have the hang of it, you can move to alternate tunings. When you change the tuning, all the chord and scale shapes change along so you need to relearn them. Typically your repertoire in an alternate tuning will be smaller than in standard tuning, but that depends on how often you use it. An example is Bach's Bourrée which can be played in standard or also with G string tuned down to F#. It's a fun exercise to learn both versions.

Here is a slightly different take.

First of all: learning the the absolute positions of the note is not all that hard. On a 21 fret guitar there are only 126 different notes you can play and only 45 of these are unique. There are only 12 different notes in the halftone scale: you just spent 10 minutes only playing C# you quickly get the hang where all the C# are. Repeat with a different note tomorrow and after two weeks you have it down.

Secondly: In many cases the relative position of the notes is much more important and useful. The role and musical function of a note is not determined by its absolutely value but by its value relative to what else is going on. An A in an A Major chord has a very different role then in an F major chord or a Bb maj 7 chord. (By the way A, F, Bmaj 7 is fun progression to play if you keep the A on top).

That's a lot what the Nashville numbering system is about. (https://en.wikipedia.org/wiki/Nashville_Number_System). Instead of writing Dm7, G7, C they would call it a II, V, I. The idea here is that a II, V, I progression always sounds the same regardless what actual key it is in. (Dm7, G7, C or Am7,D,G or Cm7,F7,Bb, etc). It's (IMO) more important to recognize relative jumps, intervals and progressions than absolute notes.

That's where the guitar comes in handy. The intervals and scales correspond to certain shapes, so you need to learn those. Again, doing 15 minutes a day on a certain scale will very quickly get you there. Same with chord shapes. Guitar makes transposing also relatively easy: you just shift the riff, scale or chord shape up and down the neck (within reason).

Once you have the hang of it, you can move to alternate tunings. When you change the tuning, all the chord and scale shapes change along so you need to relearn them. Typically your repertoire in an alternate tuning will be smaller than in standard tuning, but that depends on how often you use it. An example is Bach's Bourrée which can be played in standard or also with G string tuned down to F#. It's a fun exercise to learn both versions.