Astrologian, Dragoon, Samurai and Warrior are the only jobs that felt even remotely playable to me under 50 to the point you could enjoy them. Astrologian under 50 is good/fun for the same reason it's considering the most interactive at 80. Dragoon you have some missing pieces, but can at least do some semblance of a core rotation with two combos. Samurai you can pretty much do everything at 50 and without having to track Kenki. It's simple, but because you can do everything in the core of the job, it's playable. Warrior for pretty much the same reason.

Ninja is also pretty good because again, you have enough to at least play the core of the class but I find the job boring before 80 as a personal opinion.

The ones I specifically don't enjoy synced lower content on:
Summoner - Because buffing your pet into enkindle and contagion are no longer a thing, you aren't much of a "Summoner" before DWT, which you don't have at 50. So the job feels empty and boring.
Monk - I main Monk, and honestly it's broken enough at 80, at 50 it's even worse. You don't have form shift, so GL can and will fall off repeatedly relying on leaning on the classic PB Snap spam to get it back. Worst of all, you don't have Tornado kick, so you can't even lose GL on your terms, and because you can't form shift, it can be very difficult to maintain GL between packs if you aren't already late in Coeurl, which obviously makes you feel super slow. It's literally just the GL mechanic that causes it to not be fun, because again, you have all your core rotational abilities that you'd use the most of at 80, but you can't maintain Greased Lightning.
Gunbreaker - Feels awful at 50 and even then barely better at 60, I don't even think I started to enjoy levelling mine before 70, and even then at 71 I still don't have my AoE bullet spender.
Dancer - Only playable at 50 because you at least get Standard Step, but obvious the job is weak because you buff another DPS instead, problem is you cannot partner before 60.


Also totally get where you're coming from with BLM. Starts of simple, and clean, then turns into a total mess after enochian, then by the time you have Umbral Soul it's super fluid again and one of the most well designed and cleanest jobs to play in this game