Definitely worth considering upgrading from the Joser set. Unlike up to ML 20, you'll really need to rely on parties as a new player for higher levels and you'll drag the team down when grinding if you're not upgrading to better sets from now on. On the other hand, nobody really expects assassins to contribute to the grinding efficiency so you might be able to get away with leeching. At the very least, try to get the Osiris weapon first, as your next viable upgrade is at ML 27. You could also try farming the recipes/gears/materials yourself, as they aren't hard to make, just takes a bit of time. From there on, you can pick up the Lich set when you hit ML 27. If you have that you can skip Cazar (ML 30) and if you're still poor by that time, then you can buy Ignatius gear as a budget option (you won't be strong enough to farm it yourself) and then skip Eddga before getting to Serenia.
Also worth thinking about earning some zeny now as it is going to be inefficient relying on self-sufficiency from this point forward.
EDIT: Just remembered that Assassins do have a role they can sorta fulfill in higher level grinding: Luring monsters. In this case, you'll want to have some defense and maxed Shadow Armor, so I guess in a way, you might as well get the rest of the Osiris set as well.
Edited by Arbalist, 08 October 2018 - 11:48 AM.