concerns about purchasing a hard-shell case for MacBook Pro (M1, 2020)

Do not use cases, covers, or "skins" on Macs.

will a hard-shell case cause my MacBook to overheat?

Maybe.

Warm is normal, but give your Mac every opportunity to keep itself cool: Keep your Mac notebook within acceptable operating temperatures - Apple Support.

also, will it impair the closing/opening of my MacBook screen?

Maybe.

Apple cautions us not to use keyboard covers in particular, because they can damage the screen: Using a camera cover, palm rest, or keyboard cover on a Mac notebook - Apple Support.

should i invest in a sleeve to transport my MacBook instead?

Yes. There are many good ones on the market.

Macs are far from fragile. Having personally overseen distribution of literally thousands of MacBook Air models to lower school students, I can personally attest to their ruggedness. The most common damage is caused by closing the lid on something like a pen or pencil resting on the keyboard. Don't do that.

One might think hard shell cases keep the case from becoming scratched. That may be true, but they can also trap potentially abrasive contaminants between the shell and the case, abrading or scratching the Mac's case anyway.

John and I are in agreement.

Basically skins are not recommended for any Macs. The last Mac you could put a partial skin on was the Powerbook 1400.

But that was over 20 years ago.

The entire body of the Apple laptop is used for heat dissipation. Closing it in a skin can cause it to overheat.

A soft padded case to transport the machine is wise.

Even wiser yet is to shut down your machine before transporting it. There is the risk with the wrong energy saver settings of having the machine suddenly wake up in transit with the screen closed. That can lead to overheating.

When the hard cases become cracked, they become dangerous to the laptop since those hard cases then easily slide off the laptop. I've almost dropped computers because of these damaged hard cases. There is very little holding these hard cases to the laptop. Personally I don't think it takes much to damage these hard cases.

Another risk is you may even damage the laptop's Display when attempting to attach the hard cover, or you may damage it when attempting to remove the hard cover such as when the laptop needs repaired.