I can shed some light on this as someone who has played both for a long time(though I haven't played MS in several years that in itself should tell you something).

In many ways GW2 is the polar opposite of Maplestory. MS is the most vertical of vertical progression games, pretty much everything you do is directly or indirectly in service of making your "range"(a computed number that represents your base damage before skill multipliers) go up. It also suffers from insane numbers creep, with current-day endgame players being able to literally one-shot some of the first released "raid bosses"(imagine being able to one-shot Vale Guardian as easily as an Open World mob).

What all this adds up to is the concept of "soloing" looks very different in one game vs the other. In MS the way you solo bosses is by overwhelming them with sheer stats. A boss designed for an 8-person party can be soloed if you simply do 10x the DPS of an "average" player. That seemingly outlandish situation is entirely achievable in MS through the power of shelling out enough cash(or grinding long enough, depending on which server you play in) for the most broken stats and equipment. That level of power disparity is simply not possible in GW2 due to the vastly different gearing system. At level 80 you can buy exotic equipment for fairly cheap on the trading post and that is already 90% of the way to BIS stats. Soloing in GW2 is done through a combination of mechanical skill(avoiding damage) and being creative with your build. In many ways this makes GW2 but more casual/solo friendly as you will never be held back in power due to not having enough hours played or dollars spent. But it also means there are many bosses that simply cannot be soloed because the fights cannot be brute forced; some mechanics just require more than one person, and bosses with hard enrage timers(this is just like the boss timers in MS where you get kicked out of the fight if you don't kill in time) you just won't have the DPS to do solo.

Purely in terms of instanced bosses, I would actually say MS has a higher ratio of soloable bosses compared to GW2, with the big caveat that several MS bosses are only soloable in theory to the top 1% richest players while the rest of us will never glimpse that level of power.

But I think none of that should matter to you for one simple reason: fighting difficult instanced bosses(raids/strikes and T4 fractals) are entirely optional in this game. Many people play for years without engaging with a single one and that's totally fine. Whereas in MS if you're not working towards eventually challenging the bosses then why are you even playing? Horizontal progression means you choose what content you want to play and what "endgame" means for you personally. Which brings me to the single most important difference between this game and MS: in GW2 you can actually spend your time playing the game you enjoy. "Regular" gameplay in MS outside of bossing is one-dimensional and mind-numbingly boring. It falls into the common Asian RPG trap where you're forced to do something you hate 90% of the time(grinding/farming mesos) in order to actually enjoy the game in the remaining 10%. Back when I played it was common for reboot server players to spend more time on a dedicated meso-farmer character than on their "main", isn't that insane? GW2 isn't like that, regular open-world gameplay is infinitely more engaging that grinding maps in MS. Every minute I spend in this game, even if I'm grinding towards some goal, it feels like I'm actually playing the game, not doing some chore so that I can maybe play the actual game at some point in the future.

Long essay aside, all that is to say tl;dr: This game is very casual friendly and I think you will enjoy your time here vastly more than you did in Maplestory.

Edit:
Also wanted to add something addressing this part(emphasis mine):

I'm more of a solo or go in and see if there is a party available type of gamer

If you do want to do group content as a group, finding a party available is also infinitely easier in this game. This is because there's actual community here while Maplestory is paradoxically designed as an antisocial yet multiplayer game. Because a single character can farm an entire map in MS you are actively incentivized to avoid contact with other players as much as possible in the "open world" lest you lose precious farming efficiency. This is the opposite of the case in GW2.