Thank you for an in-depth review! Some of the things mentioned are unfortunately due to the actual constraints of the GameBoy hardware. Things like VFX would be difficult to implement without reworking how the game is built and the SFX are built into the GB Studio engine and can't really be changed.
As for combining gems to create spells, that's something that I have planned for a future update and I believe that it will really make the combat feel much more dynamic!
I've seen a few people mention the sign and I'll be addressing that among other issues very soon in a future update.
I'm very glad that you found the game unique and hope you'll come back to check out future versions of it!
Also, I wish I could give +2 just for that pun