Maximizing Battery Life and Lifespan of M1 MacBook Air (as much as possible for as long as possible)

Yes, we all understand the desire to maximize the life of our Macs & their batteries.

But you are worrying about something that does not need worrying about. All modern Macs (battery powered) have inbuilt hardware & software to optimize battery charging. Just let it do its thing. Doing otherwise will not improve anything and is likely to degrade things.

Read Batteries - Maximizing Performance - Apple for more information about this.

And, BTW, it's perfectly fine to use your MBAir when the charger is plugged into it. No damage will occur.

Do not waste your time and anxiety by micromanaging your battery use. In reality, your MacBook Air will have lost its usefulness as a computing device before the battery is an issue.

Also, in reality, you are stressing the battery more because you will be charging more often by keeping it in the 20-80% range. The most stress is caused as the battery fast charges until it gets to 80% at which point the charge rate starts to slow down thereby heating the battery more often due to charging.

Enable optimize charging and just enjoy your Mac.

Apple makes no specific mention of that, for good reason: it literally does not matter. In fact if you were to (for example) make a concerted effort to constrain your Mac's battery to always operate between 20% and 80%, always using it within that range, and recharging it within that range, you will prematurely degrade its performance and reduce its lifespan.

As users we simply cannot make a Mac's battery last longer than it's designed to last no matter how hard we try. We can certainly make things worse though. That Apple Support document describes what to avoid — extreme temperatures, using cases covers or "skins"... that sort of thing.

... and also if it's fine to use in the charger while it's charging?

Yes, absolutely.

Batteries - Maximizing Performance - Apple answers all your questions.