I read all of the first series, Percy Jackson and the Olympians. Years later I’m picking it back up. I didn’t know they all tied into each other, but anyhow I picked up the Trials of Apollo. Did I miss events that led to the Trials of Apollo?
Not really, no, you can't read it directly after Percy Jackson and the Olympians.
The events of The Trials of Apollo start right after the events of The Heroes of Olympus. It focuses on characters that you won't be familiar with, and you'll lack some essential knowledge about what's going on.
For a concrete example, the whole storyline of Leo will make no sense if you haven't read HoO.
So no, you must read The Heroes of Olympus before The Trials of Apollo.