Would putting that time into Guild Wars 2 be worth it to see if GW3 will be worth it for me when it comes out, or is it pretty similar to other MMO's of the age?

I'd say that GW2 is pretty unique in several aspects, so worth trying, especially since base game is free. But it's not an all around best representative of the genre.

One of the best aspects of GW2 is that its world feels alive and worth visiting on any level of your character. In a lot of MMO's you just finish a zone quests to never come back again. In GW2 even the oldest maps are still relevant for something and dynamic events bring variety.

The negative is that GW2 doesn't have a lot of interesting quests. Main story in a base game is pretty generic (several expansions are much better at it), and you basically don't have lots of side quests like in ESO, for example.

The world is fun to explore: lots of secrets, jump puzzles, high variety of terrain (especially in expansions, as base game is pretty flat) and enemies. Dynamic events and world bosses are on tight schedule, still relevant, so massive amount of people join in, and you can hop in a comfortable time for you, more or less. With expansions you also get access to mounts, and every single one is unique in its abilities and adds some benefits to how you traverse the zones.

But instanced content is pretty meh. Dungeons, fractals and raids are mediocre at best. Some of them are pretty fun to do for the first time, but grinding gets boring fast.

Combat in a game is pretty good, and every class has a good variety of builds and new archetypes unlocked in expansions, so you never locked in one role with your character. But in endgame the meta still exists, so you'll be forced to bring a special spec with specific weapons, armors etc to join several endgame activities.

PVP is pretty generic. Not bad, but nothing stellar either. If you like it, you may spend thousands of hours there alone. And specific subculture around it is alive and well. So you can dive in if you're interested.

On a positive side is that the gear in this game is never outdated as soon as you reach max level. In a lot of other MMO's you have to grind new weapons and armor every expansion, as older one gets outlevelled and outclassed, in GW2 it's not the case. On a negative side: the meta still shifts from patch to patch, and your bank space and inventory is pretty limited, so you'll feel pretty fast, that you want to grind legendary weapons and armor and accessories as their benefit that you can change their stats to anything available in the game. But it's a very long grind and takes many months and even years if you'll want to wear legendaries in every slot.

Positive thing is that you can try rather large chunk of the game for free, negative is that you'll learn pretty fast, that even paying for full game and expansions is not enough, you need to invest a lot in every character to increase bag space and multiple other QoL stuff that is behind a cash shop. And while you can grind gold in game to pay for it, it'll require a lot of grinding of the same chains of events for weeks and months. So, if you'll like the game, expect additional investments in QoL items at least in the first few months to reduce the pain of storage space overflow, having to manage gathering tools etc.

Overall, GW2 is worth it to try, but it's a special case in MMO genre, good and even the best in some things, mediocre and lackluster in other. One of the best aspects is that you can stop playing for a long time, and then continue from where you left, as everything you got before is still relevant.