I was on the Manta Queen 8 in 2019 and the South Siam (Somboon 3) in 2018, both in the Similans. I'd rate them as adequate; the trip was definitely worth doing, but the boats were nothing special. I knew this when I booked, but it came down to timing and availability.
To the OP, unless something has changed since COVID, liveaboard bookings are like the old days of travel agents in the US with Thai dive shops (or liveaboard.com) acting as agents. The agent get a commission from the operator if they book it and even if you book directly with the operator, you still get the same price.
For this reason, your might as well talk to a local shop that knows all the boats and can help you match your skills/desires with an appropriate op. I was very happy dealing with
@stevenl of
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The main exception that didn't take bookings from unaffiliated shops was The Junque/Phinisi. I don't know if this is still the case, but if it is you'll want to evaluate them separately.