If you like your current speakers consider powering them using a Sonos Amp. It is usually good enough to power even larger speakers to reasonable levels. You could also upgrade to other passive speakers if you like the setup but want better sound.

Don’t get your heart set on a position for the Sub, it is really best to try several locations and pick the one where it provides the most even sound across the room. I’d say a full Sub for that sized room.

Keep in mind the Sub will only bond to one Sonos Room, be that a single speaker, stereo pair or surround set. It will alter the sound of the speaker(s) it is bonded to so you’d get the most boom for your buck by bonding it to your weakest (on the low end) speakers.

I’m pretty happy with my Arc for music but the stereo separation is pretty weak out of the sweet spot. A well positioned pair of Fives gives a much lager sweet spot.

Once you start Grouping, adding a second set of speakers your stereo image can be a problem, some have no issues with that, other folks find it doesn’t suite their ears.

I actually need to replace the speakers too as my kid managed to poke a hole in one of the cones :(  - So Amp plus new speakers will come in around the same price as two FIVES 

Do you find the Arc position is critical? If i had an Arc it would be in the corner of the 8m wall under the TV, rather than centred on the 4m wall facing the stereo pair on the opposite 4m wall. If the positioning isn't critical then this might be a good option. In that case I would go 

x2 FIVE 

x1 Arc

x1 Full Sub  

For god Atmos sound you need to be pretty close to the Sonos recommendations for location, for just stereo or L/C/R TV it is a bit more flexible but the end speakers do need room or the stereo image will suffer.

I’d bond the Arc and Sub in that situation.

For non-TV audio casual listening you could easily Group them all for better room filling.