My own personal choice, given that size of room, would be to go with your option 2. The Beam (gen 2), the Sub and my preference would also be to add the 2 x One SL’s aswell… it can be achieved either by purchasing the sub/surrounds at different stages and spreading the cost, or all in one go, which would be the cheaper option, as per this link:

https://www.sonos.com/en-us/shop/5-1-surround-set-beam-sub-one-sl

If you are interested in Dolby Atmos audio and want more convincing height channel effects, I would go with the Arc + Sub. The Arc has two upward firing drivers. The Beam (Gen 2) does not. And I believe adding a Sub is more important than adding surround speakers since the Arc will produce a fairly convincing virtual surround effect in a room that size.