Here is the solved ‘hard problem’:

Basically start with the pair of 20s and you will find only these pair of triangles work as a configuration. And work anti-clockwise from the 3, 4, 12 on the right, 12 on the right, the 5 etc.

Here is the solved ‘hard problem’:

Long story short I started with the pair of 20s in the bottom and found that only these pair of triangles work as a configuration:

This then forces this since the 3 triangle has to be positioned this way to enable a 12 triangle to be formed…

Which then forces this

The 5 has only one way to be formed… Leading to this:

The three triangle has two options but one of which doesn’t allow the 5 triangle so it forces it to be like so: After that there is only one way to complete the grid.