I'm a Bronze 5 Support/Jungle main who can't play any assassins besides Yi and Akali (and I'm bored of their terrible early game.)
Are there any challenging assassins besides Zed, or is he the only one I can have fun with without slamming my face at my keyboard, repeatedly, and get a pentakill?
If you want something easier than Zed, isn't Akali, is an assassin, and can string together a Penta... Bilgewater Katarina 
. I would've said Diana because I love her, but she's not going to get you a Penta as easy as Kat will.
Vouch for Katarina.
maybe a fizz but since he banned in bronze a lot, then try to get talon but doubtfully u will get pentas, maybe a quadra if you are ahead. There are better assasins for ex. khazix would be a good jg assasin, that can get a penta then try get le blanc
, yasou 
are good champs in solo queue
If you like a challenging assassin, try 
. His mechanics is one of the hardest to master in the entire game. I practiced him a lot on normal games before taking him into ranked games, and he carried me from silver 4 to plat 4
I mean, pentakills usually only happen for snowball-y champions like ADCs or assassins like Yi, Akali, Kassadin, who have weak early games. Pentakills usually come from DPS, not burst (which is what most assassins like Zed or LeBlanc do). Check out Katarina, Yasuo, Riven, they're pretty challenging to master and you can probably get a penta with each of them (Riven and Yasuo aren't as bursty as the other assassins, but they have good DPS and high skill ceilings)?
Katarina is the most fun champion in this game, being able to Jump around at the speed of light in a team fight is insanely fun and so addictive. Katarina has the best passive in the game, this is what defines her and makes her fun at the same time. Survival for Katarina is tied to her passive, so is the vast majority of her offensive power. You have to get the kill or a assist to survive as Katarina, there is no dissapearing into the shadows like akali or zed and Kat does not have a troll pole on a 5 second cooldown.
Picking up Katarina is kinda like putting, easy to pick up and have marginal success with but takes practice to master. Katarina, unlike other assassins is a greedy champion select pick. Katarina is team dependent, you need your team to start and for the most part be winning the fight for Katarina to excell. Virtually everything counters Katarina in lane, however you can max q and still farm from behind your turret and stay relative. You must know how to roam to play katarina.
Warning, Katarina game play is addictive.