Ah - work form home so no real ties. You could live in the middle of nowhere (but do check available broadband because it can still be an issue!).
Again - you don't really give a lot to go on with preferences though... The beach being one thing. On that: Chesil beach is not great for swimming off due to currents so stay away from there if that matters.
I like Lyme Regis (grew up nearby) but it gets really busy in summer. Charmouth is quieter and I like it too. I'm not a fan of West Bay personally (seems over-hyped to me since that TV series) and large caravan parks. Bridport seems to be a thing for Londoners escaping these days so is a good suggestion from a previous poster. You'd drive (or cycle?) to the beach.
Countryside from there to Weymouth is nice, Chesil beach has its charms and challenges.
Weymouth centre is a bit shabby, but the town beach is amazing for family friendliness. Plus the bay is good for water sports (kayak, sea swim, sail), has Parkrun, a cinema, pubs by the harbour. I have family in the Preston area. That is retirement-ville but nice, probably not what you want though.....
Dorchester is the county town but is still quite small. Good sports centre, a hospital, shopping streets. Not multi-cultural, but does have culture. Need to drive to the beach.
Swanage area (and the Isle of Purbeck) I really like, but not a multicultural area.
Poole and Bournemouth might suit you - but they are city beaches rather than proper countryside. East of Bournemouth gets smaller again with the New Forest.
Bournemouth would be most fun for teenagers (it's a university city with quite a few clubs).
Lots of good walking, cycling and horseriding about in the green bits of Dorset. Even a dry ski slope!
Think about what is most important to you to have on your doorstep, traffic in peak tourist season is a PITA.