Having been soloing through most of the story, my biggest complaint about early bosses is entirely their health bars are insanely long.
The difficulty some might complain about, though personally I find them on the easy side. This is expected- it’s early level bosses that are supposed to teach you how bosses work before ramping up and politely suggesting you bite the pillow before going in dry. As for why I find them easy, I play Yunica in Ys Origins, play Touhou on Normal at least, and do some soloing in Vindictus (think: Monsterhunter MMO). I honestly find the attack warning markers “quaint” and unnecessary minus the hard-to-track random parabolic attacks and the sudden pillar nuke attacks.
However, currently, when soloing, the biggest challenge to me is boredom. I started looking for parties not because I needed the help, but because it was significantly (re: at least 50%) faster to have someone else with me.
What I suggest is, since quest bosses are instanced, how about scaling their HP based on party size. The smaller the party, the shorter their health bar.
The current bars feel like they’re probably ideal for 3 ish people. Lowering it to about 60-70% for solo and 80-90% for duo would probably vastly improve the early game experience (at least until you hit the “why aren’t you in an organized party right now” quest bosses).
Now, whatever you do, don’t increase the HP 1:1 per party member. That’s a great way to make bosses a total slog- especially on a burns-brightest class like Wizard, who will start to resent that boss fights go faster when they solo.