Maybe you have too much squish?

Wipe down the build plate regularly with IPA to help keep it clean; I do this as part of my pre-print workflow. That and scrubbing the nozzle with a brass wire brush to help keep burnt filament getting into my print.

How hot is your shed? Depending on your shed temp, some filaments actually like it hot. I have a PLA filament that likes to print at 220C and in an ambient enclosure temp of 90F.

If your shed is near 120F-140F, its perfect for ABS/ASA (be mindful of VOCs); maybe consider switching filaments while its hot out - making lemonade out of lemons and all that.

cheers

Here are some things to try:

• make sure you have properly cleaned the new bed; ipa or dish soap. I have gone so far as lightly sand my new pei sheets with high grit sand paper to improve adhesion. Try the list below before considering sanding.
• bed temp: as you mentioned a cooler bed temp may improve corner curling. 50c or just yolo it and turn it off
• sounds like you’re printing with PLA: maybe slightly lower your hot end temp.
• mouse ears at the corners may help keep them from curling.
• make sure your printer isn’t catching a cool breeze.

FYI. The Bambu P1P is currently going for $599 USD. It’s a core xy , with impressive print speeds, with an upgrade path to a proven multi materials solution.

Since you are placing it near an AC, you’ll need to print the side panels to partially enclose it. Lots of community upgrades available.

Good luck.

The cost of a 3D printer is more than just a printer. Other things to consider are the consumables like filament, periodic replacement parts and a computer to do the slicing.

An idea on getting a low cost printer; The Goodwill Store in your neighborhood sometimes can have a 3D printer for very cheap; these are usually from hobbyists that lost interest in 3D printing and just wanted to get rid of the printer. Pricing may be $50 or less but may require repairs/parts.

A no cost alternative to getting started in 3D printing is to contact a local library or if you still have access to a school makerspace, to see if they have a 3D printer you can share time on - sometimes its free and sometimes its a low fee to rent the printer time.

I have a Prusa Mini original launch edition, and can’t say enough good things about the printer - we run a low volume Etsy shop with the Mini and it’s been a reliable work horse.

One thing I learned that is not on your list is that Prusa has great customer support; live people that you can talk to on your questions or issues. This in my opinion separates Prusa from most other consumer printer companies.