Personally, I don't really believe in working as hard as you can, earning and saving as much as you can in a shitty job, to get to FIRE asap. If that full time job made you feel burned out, then that's not a healthy thing. And no amount of FIRE is worth that. Health goes above everything, because, without it, your ER won't be as good. I'm not an ultra hardcore mustachian as some people. I don't necessarily want to achieve FIRE the quickest I can, but I want to be HOT. So I save and invest, so that I become a little bit HOTter each month. And it allows me to work part time and enjoy my free time already. I like that.
So, in the case you're describing, I would say, working part time is perfect for you now. If you're happier now and still able to save (albeit a slower progress) than what's wrong with that? Although some people like to pretend it's a race, it's actually not. It's great if you manage FIRE, but if you manage it in 5 shitty years, working much too hard and leaving you burned out. Then it's much better to take twice as long, if that means that you'll stay happy and healthy. And maybe in one year or later (or never) you want to work full time again. You can still switch back then. You're still working now and gaining experience.
So, my advice would be to enjoy it. And maybe one day you decide you'd like to work full time again. And maybe you like working part time this much, that you keep working part time. You'll find out. Just enjoy it for now and reevaluate in a year or so.