I think if it's taking you over a year to get a two part invention together right now, then you should consider backing up a bit. You're bogging yourself down with unnecessarily harder work than your present level. Soon enough your hands and mind will get back in shape, meanwhile work on things that you can exercise both in without a big struggle. So back up in level of material, keep one piece a bit more advanced to put your extra effort into. The fact that you like the carols and enjoy them mostly because you can play them supports my thought on this.
I've done the same thing myself since I returned to piano ( and funny, I've found joy in playing carols and hymns as well). I was away a really long time, figured backing up to level 7 work would do it for me but ended up a few months on level 3 work and presently more stuck around level 6. Ya I can do level 7, have done a couple but it's a battle I wasn't ready to take on, never mind beyond that. Like you with your invention, it just takes longer than it really should. So I found all sorts of music I have enjoyed and even performed ( for family and friends), that I could get together in less than a months time, then spend a couple of weeks perfecting them before I played them realistically for some one.. My level is slowly rising, I did have some medical issues that slowed me down last summer and fall though. Even so, I'm pleased with my progress and I'm pleased to just be playing again. Coming up in late April to May, I will be back at the piano now for two years.
The key to all this , enjoy yourself !!!!