I would also suggest you try for a Tubbataha liveaboard in the Philippines during that time. You are traveling within the short time window that it is open (late March to early June) with the ideal time being mid April to mid May. It is arguably the best diving in the Philippines. I enjoyed it, and I also really liked Raja Ampat and Komodo (my 2 favorite places for warm water diving). Many whale sharks that swim by naturally there. Lots of sharks. Jesse Beazley reef is beautiful.
I dived the Similan Islands via liveaboard in April 2019. It was okay. Richilieu Rock is very good, but outside of that, everything else was pretty underwhelming. It's fine if you have never been, but I find the diving in the Philippines to be much better overall.