I think it looks really cool! The shelf is a unique touch that will make placing corals really easy. Since it's all connected your mushrooms or zoas may eventually try to take over the whole structure but that should be pretty easy to deal with especially in such a small tank.
Only thing I'd change is actually the light. I have one of those Fluval minis and find it to be really underpowered and too heavy on the whites. I use it on a 4g planted shrimp tank now and it's perfect for that at 100% brightness. It would probably do ok with shrooms and zoas but not torches and gonis. Here's a par reading in the 4g cube, all channels 100%, dead center, 8" from the light (3in air, 5in water):
View attachment 81143
I'd recommend the noopsyche k7 mini as the perfect light for the IM 15. It actually costs around the same as the Fluval but is way more powerful and better spectrum.
https://noopsychereeflight.com/products/k7mini
Here's a video of a guy doing par reading with the K7 Mini on an IM 15:
Btw, I have a build thread for the IM 15 with some custom 3D printed parts if you're interested. I've already printed a refugium for 2 other members