Era 300 is designed to give a better spread of sound than the 100, but at a cost. (There are other differences too.) 

Sonos does have a good Returns period in most regions so buy a 100 and a 300 to try, and return one if you think it’s under/over spec for your needs. Then add the extras for the other rooms. 

That would be a very nice setup and you should be very happy with it.

You might consider:

Adding a second matching speaker to the bedroom or office if you can position them for Stereo sound and spend a reasonable amount of time sitting in the sweet spot.

or

Going with a single Era-300 that will give you Atmos sound.

https://support.sonos.com/en-us/article/listen-to-music-with-spatial-audio-on-sonos-speakers

I spend several hours a day in my 10x10 foot office and I decided to go with two Era-300s, the pair are really more speaker than the room needs but I got spoiled listening to Atmos music on my Arc and decided the “cost per hour of listening” over the speaker’s lifetime was going to be pretty low for the size boost.

I spend some time in the 15x15 open-plan kitchen where a single One does a more than adequate job of filling the room at background levels.

I also have a Play 5 (gen 2) that I use by my recliner, the second most used listening spot, and it is way too much speaker for the space but it offers limited Stereo sound so I prefer it to a smaller mono speaker and I don’t have space there for a pair. It will be replaced with an Era-300 at some point so I get Atmos there too.

If you’re adding to the Arc Ultra to get surround in the living room, either will give great results. If it’s just for music, the Ultra isn’t too shabby on its own.  Move2 for the deck should be fine.