Here's some basic before/after examples that might be useful. This is kind of a deep rabbit hole. My dumb brain still dreams of blackbird studio c every once and a while.

https://www.youtube.com/watch?v=dB8H0HFMylo#t=6m22s

Build your own panels with rockwool insulation. There way better than almost anything you'll find on the market and easy even for no talent carpenters like myself

Exactly. If you really want to get into it the depth of the construction of the panels you need is based on some maths - density of the insulation material and it's particular properties but all of that can be found out on forums such as this one: https://gearspace.com/board/studio-building-acoustics/ - vs the frequencies you wish to treat.

You can put your room dimensions into a calculator and get a rough idea of some of the try and find the modes of the room - which you want to treat, though sometimes it takes trial and error as well. But you want to treat the point of first reflection and then have bass trapping in the corners.

Don't buy that "acoustic foam" that looks like egg cartons, it's rubbish.

How do you cover those? I'd expect drywall or similar would negate most benefits.

Fabric that you can breath through easily for looks. Under than you can be very thin fabric that makes sure that the fibres from the insulation doesn't escape.

I use this for looks: https://www.camirafabrics.com/en/contract/inspiration/acoust...

I've found https://ehomerecordingstudio.com/acoustic-treatment/ to be a well-written and informative guide.

You might look here: http://realtraps.com

You can probably reduce some room resonances.