Easy but complex, free, short sessions? Sounds like a job for tetr.io

ShellShock Live. Fits every one of your criteria.

Have you played 'Vampire Survivors'? Rounds last at maximum of 30 minutes Easy to learn, but there's a lot of content to keep you occupied It's $5 but it goes on sale often

Brotato

Not exactly a "game", but a collection of games -- XCloud. And yes I know "Microsoft bad", etc but its an awesome choice for extremely low powered devices like the Orange Pi zero 3. (its power consumption varies through 1W to 3W.)

t. Am typing this on mine (1 GiB) and it runs extremely well (60 fps, with almost unnoticeable hiccups.)

If running it on proton is an option I would recommend outer wilds. Fantastic in every aspect!

Factorio

Your Only Move is Hustle - Basically a TAS Fighting game, the game plays like chess meets a fighter game, you preselect moves at the same time as your opponent, then they play out. Sounds super simple when explained like that but there are so many complications to this basic formula (cancels, bursts, DI, parries, to name a few), that it is actually very interesting.

Omega Strikers - Air Hockey meets MOBA, although the game is dying a little, but the devs are still active and they just released a new character, their design never disappoints and the OST is fire.