Guild Wars 2

Title. I started a P/P and enjoyed it, but found the playstyle a bit bland. I went to Mesmer and found it fun, but super ultra squishy.

In terms of overall fun, solo power etc, which would you advise and why? I’m interested in both PvP and PvE.

Both these classes require some time and effort to be able to fully master them. For the mesmer people say that from about level 40 it becomes much easier and more fun. Also as I play a thief myself, I would recommend making full use of the stealth mechanic! With most proffesions its all about making the most out of their special mechanic. If you are able to control stealth properly, with different sets of weapons ( I love the sword #2 skill) you will have a lot of fun and power!
I don’t know too much about mesmer, but I can always advice you to just play and try everything out! The best way to find out whay you like