gssr, help a novice ^^

First advice would be to level up lowstars that you have, it will help going forward.
Servants like Euryale, Robin, Arash, Medusa ..etc can be a solid addition to your team.

Regarding the GSSR, Jeanne is good servant to pair with Martha so if you roll that banner, you'll have either Jeanne or Jack, I'd say a win/win situation.

Going for Saber is not a bad idea considering all three options are AoE and solid servants.

I'd recommend going for either the Assassin/Extra banner , the Saber banner or the Lancer banner.

Pick caster banner having a good support goes along way. You could also go for the Extra/Sin banner. Both units in that banner really strong one at being a tank, and the other at making stars. Saber banner is pretty meh imo, so i wouldn't go near that one.

Some things to consider:

Rider Kintoki and Archer Nobu are solid welfare servants coming soon.

Saber banner is meh, arguably the sr sabers on that banner are better, saberlot surely is.

Lancer I'd only roll for Karna, maybe Enkidu. You don't need aoe though since you already have jalter lily which has np battery and is good for farming.

Caster banner is probably one of the best banners. Waver is top tier, Tamamo is very good and Xuanzang is probably the best caster on her niché. The best pull would be Waver and Tamamo on second place both are very good.

Nothing more to say about the assassin banner that hasn't been said already.

As for the berserker banner you already have Heracles which is one of the strongest berserkers so I'd say skip that one for sure.

Merlin goes best with Herc, or Waver depending on your needs. Stheno can also give Herc an extra buff because he is divine but that's kinda gimmicky. OG Saber and Altera aren't really suitable for Herc. If you want to put the three of them together that's fine, but there isn't much synergy between them other than having lots of buster cards.

To be objective with gameplay in mind, Caster is still the best banner for you by far. Waver alone is worth it, Tamamo, while tied specifically to arts teams, outperforms every other servant at what she does, and unless you plan on maining Herc forever, having Tamamo opens up a lot of options for you. Xuanzang is also arguably the best in the small but valuable niche of caster DPS.

That said it seems you don't care for that banner, no reason to roll it if you don't care about any of the servants. In that case, just go with what you like most. You mentioned Altera and OG Saber - if you want them only because you think they'd make a good team with Herc, I wouldn't roll - as I said, there isn't much synergy in putting 3 damage servants together. If you genuinely like and want them though, then by all means roll saber.

Rider and Assassin are both great banners. With rider, 75% of the servants in it are amazing, with special mention to Drake being one of the best AOE servants in the game, which you lack. With Assassin, both servants are good, you really can't go wrong with it.