Alright, I guess I'll soldier on for now then. Thanks!
Note that the list isn't actually accurate. You get the xp bonus as long as you have one class higher level, it doesn't have to be 60, or 50. It can be level 2, and your level 1 will gain bonus xp(well technically there might be a minimum, but it works at 30 so it's low). It's also 100% bonus xp for 1-49, then 50% until 60.
The seals are only unlocked when doing content with the max level, content that can be used to level up(low level duty roulette, then for 50-60, high level duty roulette, though that needs to be unlocked, as well as trials). But unlocking it really doesn't do a whole lot, especially since all it unlocks, currently, are uncapped seals. So you get the EXDR daily which gives you 80 laws a day bonus, but that's just one more run, it won't make a huge difference. Unlike capped tomes that you'd want to cap every week asap, laws you can farm as much as you want. Capped tomes are coming though, in about 2weeks, so it might be short to make it.
Retainer is right, but depending on how much you play and how far you've gotten, your retainer might be quite a bit behind anyway. Hell if you haven't started, it doesn't even matter. If you did make an archer retainer, you will need to level archer to 60 eventually, or respec your retainer.
The more important thing to consider is wether or not you like your class. If you do, you might as well keep going. If you don't, the earlier you roll something you like the better. The content isn't running away, and it'll be the only content to do for the next ~2months anyway, so there isn't much of a hurry.