M1 macbook air battery health

Yes that's normal. Battery capacity is a non linear progression; it can sit for months, even years, at a given lower capacity. 80% is where Apple advises you replace it.

It's wasted effort to micromanage your battery. You Mac's charging circuitry will take far better care of your battery than you can. Charge it when it needs it (never going below 50% is not being especially kind to your battery as in limiting charge to any specific value not doing it any special good), use in battery mode when you need to and enjoy your Mac.

You're way overthinking this.

Rechargeable batteries can lose some initial charge quickly and the last for a long time at that level before losing capacity, some will hold initial capacity for a long time and then start to lose capacity, some will start slowly losing capacity from day one, and any other possible combinations.  It is just the nature of rechargeable batteries. Micro managing your battery use will nota alter any of these facts.

FWIW, don't try to micromanage your battery and just use the Mac.  If you reach a point in time where the working time on battery is not long enough to meet your needs, then start to worry or just get the battery replaced.