I always feel guilty wasting perfectly good papers

Lol, me too. But if you can, watch some older artists on Youtube: more expensive papers just make certain things easier. Cheap papers make the same thing harder – NOT impossible.

That’s how cheap paper builds your skill: you’re forced to pay sharp attention to the dampness of the paper, the exact sheen of it in the light, to know when it’s ready for a blend or a scrape (where you scratch into wet paper to either create dark lines if it’s wetter or remove paint if it’s drier), etc.

I don’t know if you’re in the US but the paper I like most for watercolor is also one of the least expensive: Canson XL watercolor paper. It’s a nice, heavy 300g that takes a good bit of abuse from scratching, scrubbing, lifting, and pretty much anything else I want to do. ALL watercolor paper will buckle, but the heavier weights keep it to a minimum, and 300g is an all-around great weight (heavier weights are professional use and tend to get really expensive). Arteza I have also heard good things about, inexpensive and good, but Canson’s usually at the Walmart so I can just get some whenever I’m there and avoid Amazon, lol.

For painting boards, I save and use the clean tops of pizza boxes, lol. Any clean stiff cardboard will do, turn it over and use the plain backside so there’s no visual confusion, and cheap masking tape or washi to hold it down if you don’t feel like freestyling it, lol. For the copy paper practice I wouldn’t even worry about that, any tape you want to use. (You can also use scotch tape if you weaken the hold on it a bit by taping across your clothes before you put it on your paper, if you’re in a pinch and need a removable tape.)

Your watercolors came in a tray; that’s also a paint mixing tray. Take the paint tubes out while you’re painting and then rinse it afterwards to use for tube storage again. And use any old plastic container for water, etc. Now you’re all set!