Or is it better to buy: Apple Homepod 2nd gen for this set?

I would personally purchase the Era 100s over a Sub Mini as your next addition.

The Era 100s would provide surround sound which would elevate the audio for both movies and music listening.

The Sub Mini can be a future addition if you wish more bass.

Jeśli mam Apple TV 4K + SONOS Beam 2, to nie ma sensu kupować Apple Homepod 2. generacji?

Ps

Regarding the SONOS ERA 300, I read that the front speakers are disabled with Dolby Atmos enabled.

Is the ERA 300 worth the extra money?

Atmos is a positional codec, and is not mapped to any single speaker or driver within a speaker.  Therefore, what drivers play a certain sound depends on the mapping of your room.  And since every room is unique, reading about what drivers are/aren’t playing in someone else’s room is useless.  You either believe the mapping is doing it’s job, or you don’t, and suffer with the OCD notion that what you paid for aren’t playing at all times.

Dobra, wracając do tematu: który będzie lepszym wyborem dla mojego zestawu?
SONOS Mini czy SONOS ERA 100 x2

You have to pay a lot extra for the ERA300 and the room is only 15m2

As you already own the Beam (Gen2) it makes more sense to expand the Sonos system that it does introducing a separate sound system.

The front speaker on the Era 300 is disabled when used as a surround speaker, irrespective of the audio format being played.

Whether Era 300 is worth the extra money over Era 100 is entirely subjective.

Dobra, powrót do tematu: which one do you recommend for my set?
SONOS Mini czy SONOS ERA 100x2

Za ERA300 trzeba sporo dopłacić, pokój ma tylko 15m2

That really depends on your tastes.  Do you like big booming effects, or more subtle, directional or atmospheric effects.

Which of these two models is the better choice for watching movies and listening to music, offering the best surround sound quality when paired with the Bear 2 soundbar?

SONOS SUB Mini or SONOS ERA 100 x2?

Which one do you recommend?

They are two completely different options.  The Sub gives more bass, the surrounds give atmospheric and directional effects.  Only you can decide which are more important.

If I knew, I'd choose it myself, but I have my doubts.

A subwoofer for more bass = SONOS SUB mini
Surround speakers for atmospheric and directional effects = ERA 100

Help me choose.

I’m sorry, but I can’t help you choose, beyond telling you the differences between the two.  I don’t know your personal likes/dislikes. 

Let me ask another question: if you were faced with such a choice and had such a set, what would you choose?

I can’t say.  For the two Sonos soundbars I own, I purchased both a Sub and surrounds for each.  

What subwoofer and surround speakers did you buy?

I have a Beam 2, Sub and One SLs for the bedroom, and an Arc Ultra (formerly an Arc), Sub and One SLs for the living room.

subwoofer masz SUB mini?

It appears the One SL is no longer available for purchase.

Era-100 is the replacement for the One series.