"Please bear in mind that the target for this game is the Game Boy Advance and 386 DOS machines, so please judge the graphics and UI/UX accordingly (my custom engine is not up to spec to a Unity or Unreal, of course!). "
This statement has me wondering why you chose those platforms. I get that some of the chips that used the 80386 instruction set had a comparable speed to the GBA, but in all other respects I'm aware of, they're not even close. That even includes what type of processor is being used, since 80386 is x86 while the GBA's processor is ARM. It's hard to even compare the video capabilities, since I'm pretty sure the use of 386 processors wasn't aligned with the use of any particular video standard. Just looking, I can't tell if this is using CGA, EGA, VGA, or one of the lesser known ones.
Also, in case you're not aware, both Doom and Duke Nukem 3d got GBA-ports, so that's what the graphics and UI/UX standards for comparison would be.
Having tried both versions, I'm unsure why you allow free movement rather than something more like Myst or 7th Guest.