I mean...he's already a fully-fledged muse so,
1. 1st Footman
2. 2nd Footman
3. Valet

I think I am leaning more and more towards the Sanditon for reasons already outlined! We can still do the joint-NPC-headcanons for the permanent guests for those who prefer it, and we would definitely get a lot more interaction with played characters with people visiting/our staff getting hired out during the season. The Sanditon events are already established so we wouldn't have to host them through a NPC household; the Sanditon staff presumably also have a little more freedom of movement than in the one household where many of them are not otherwise going to be getting out much xD And as the resort has probably a bigger staff pool than other places, if people want to join later there's probably more room for them to do so?

But still with all the good domestic service related drama, of course. xD

Also the Downton downstairs went to the beach once as a group outing so I think it's cute.