Berserkers

Imo the only other ST zerk that is worth levelling over Herc (at least for their gameplay value) is Cu Alter. Not to say others aren't good, but in all honesty Herc is just about all you'd need from an ST zerk. If you want to level others there's no harm, but if you wanna save the mats/QP for other servants you won't be missing much.

I just got Herc to 85 farming today's quest and I was wondering what to do with the extra xp too. I don't think I wanna grail him again yet, so I'll probably use the XP for my Jalter or Salter santa when the xmas rerun starts.

Yep. There's no point in leveling another. You only need at least an AoE and an ST servant for each category. Though there are some instances such as events where you will actually need to have leveled up some servants since the ones you have may not be usable just like on the Last Halloween Event.

Definitely focus one character per class at a time. The progress is too slow after level 50ish to be levelling two servants of same class at same time.
It depends what team composition you're working on. I've been playing since March and I've got np2 Vlad who works better for arts based teams with ST NP, Minamoto-no-Raikou for farming and living a bit dangerously with her AOE NP who has 2 arts cards, Herc about a month away from bond 10 as my trump card and cu alter who I'd argue is the answer to all situations. I would strongly suggest if you summon cu alter, level him as a guarantee for difficult quests. My current project is Fran and one day getting a kaleidoscope.
Herc is a great berserker and becomes much better with his bond 10 craft essence (which will take 5 or 6 month I think) so if you do only want one ST berserker, he's a fine choice.

Always worth to raise as many as possible because it will be very boring to just using the same 1 or 2 characters for years no matter how strong they are.

Herc alone is enough, but if there's a husbando/waifus berserker then raise her/him.
Sekai no Ichiban Tsuyoi Dakara 💪

Kiyohime/Darius/Eric is a good AOE wave clearer and farm zerker. If you lacking farm/daily servant then they can do their job well.
While Herc doing another job like 1vs1 duel with bosses, Last Man Standing/anchor even can be used as Buster Crit ST Zerker but Anchor is his best job.

From gameplay perspective Hercules/Chu Alter is very strong.
Zerker like them is a really good medicine to counter lots of classes, even among berserker.
Most berserker are really easy to kill, but Herc/Chu Alter is an exception.

He is really great for long term Investment, i would say just stick to Heracles.
Though it's up to your arsenal condition.

i guess you can still level more niche st berserker like vlad for particularr content, or strategy. but unless you find a cu alter (like me) or someone you like more, he is good

After raising Heracles, why would you level any ST unit at all? .-.

I'd say level Spartacus if I were you. His skill gems and ascension mats are really cheap and he has a good NP charge + buster buff skill letting him hit for 30k+ per enemy. Idk what other Berserkers you have so this is who I'd recommend. Also depending on your Hercales' NP level Lu Bu might be worth looking in to. He has one of the strongest hitting NP, outmatching almost all the 5 star servants and only losing to a select few with niches against certain enemy types.

It is definitely worth it. I have Ibaraki as my main ST berserker, but I also have a maxed Kiyo to help out with the daily quests. Now that both are maxed, I am slowly leveling Tamamo cat using the exp cards from the daily quests. It never hurts to have more than one good servant for the same class, as they all shine differently. There are hardly going to have times when Herc won't be useful, but there ARE times where you could benefit more from someone else's skills, cards for chains, and so on.

However, as people have already mentioned, do not focus on more than one servant at the same time. At the later levels, raising a servant costs way more resources than you think. Max Herc before you consider leveling other berserkers. He's definitely worth the investment.

There are a few times in the game where it can be nice to run a team full of berserkers and just buster blitz your way through some stuff... For example, when you're running the easier quests in an event, the mobs are often of mixed types and have low enough health that you could one-shot them with a buster card and class advantage. So you can often just take in a team of zerkers and get through those types of quests in 3 turns, without having to change your team around. In those types of cases, it could be nice to have another zerker to go with your Herc, but it's not any type of priority.

Also, you will probably want to raise an AOE berseker or two for farming at some point. Spartacus is highly recommended, and Kiyohime is alright.

Ideally, have one ST Zerc (Herc and Cu Alter being the optimal choice), and around 2 AOE Zercs for Farming (MLB Dragon's Meridian on each+Waver can speedfarm any Chaldea Gate Quest) and Wave Clearing harder mixed nodes.

For AOE who are free, Kiyohime, Darius, Eric or Spartacus are efficient. Do note that Spartacus is much weaker, but he has a minor charge NP, so with Kaleidoscope/MLB IE he can wave clear by himself (except the hardest nodes). Kiyo, thanks to her weaker pseudo-mana burst+AOE low stun chance, is quality good for farming.

for ST, Cu/Heracles
for AoE, i can think only of Raikou
for support (YES, there IS a single support, and quite good) - Nightingale

well, last Hell event had me raise up that Banana Oni, and she is decent too as ST

cu alter + herc + waver can carry you over all the game