Input for best homebase for relaxing sea holiday

I would consider it too crowded (I did visit in late May, several years back). I have not been to Sestri Levante.
I have enjoyed Lerici and Camogli during this same time of year. Quiet but not too quiet, ample hiking without hordes. Better beaches at both.

valadelphia is right. Camogli is quite a bit further north, but we very much enjoyed a week there last fall. You can get to the Cinque Terre by train for a day trip if you like. Lerici is served by bus and boat only, no trains, but (though we have only day tripped there) a very pleasant town. Friends of ours liked Tellaro but again, bus only, no trains, but they had a car.

We have stayed in Santa Margherita Ligure twice, near Camogli. We love it there., walking, boats, good food.

Anywhere in the 5 Cinque Terre villages is crowded with tourists (albeit not at the beach, since there is no beach except for a small one in Monterosso, but certainly at the restaurants and hotels. Moneglia is cute, not far from there.

All other towns on the coast will not be crowded in May and the beaches largely deserted because it’s generally too early (and cool) in May in northern Italy.

I would consider the Amalfi coast instead. More to the south, therefore more likely to have warmer weather, and if not more things to visit even if it’s not beach weather.

It can be very warm in late May, but that's not guaranteed, especially at the northern end of the country.
If you want a beach location within reach of Rome but with cleaner water (more appealing for swimming) than the Ostia area, Tarquinia has it, including lockers, rental umbrellas, etc. There are also the Etruscan archaeological ruins to see.

Depends on if OP is looking for just seaside or actual swimming temps, but I agree--the sea is usually too cold for me in late May, with the notable exception of southern Tuscany--I theorized the seabed may be more shallow there. Of course it also depends on where you are from--I like warm water for swimming, but Canadians might tolerate the sea temp better for example.

Have you considered Sicily? The weather will be better for your desired activities.