It'll cut much bigger than 1/4" ply, that's been proven. 3D printed parts are also a lot stronger than you'd think. A quick google search on the lowrider2 or MPCNC will yield plenty of results and videos.

This guy, well known in the 3D printing community build one and has cut aluminum with it. https://www.youtube.com/watch?v=R7Tk6wY_9ms&t=491s

This guy cutting 3/4" stock. https://www.youtube.com/watch?v=vkHcQwobaSs

Here's another from the designer himself. https://www.youtube.com/watch?v=GWD2uv0K4IE&t=87s

3D printed parts isn't the limiting factor, it has more to do with the size bit and router it's able to carry.

Again, there's a facebook page with over 4k members and countless post of people sharing their cutting videos and finished projects.

No one is saying this is better then having a manufactured CNC that's store bought, but it's focused on the OP's desire to find an affordable CNC router and this fits the bill. We run 3 CNC routers at my company, all 3 north of $250k a piece. I'm not saying this $600 mostly 3D printed CNC router can ever come close to the ones we run at work, but it will work and it will cut, just slowly.

As for the belts, they run just fine. Carbon reinforced belts don't stretch like solid rubber belts and hold up for years. They're the same belts we run on our 3D printers but wider and thicker.