Should I make some SQ rolls for more servants?

As always disclaimer: roll with saint quartz on banners that have servants you'd want for any reason, waifu, gameplay or otherwise.

Moving on,
For gameplay purposes you can afford to save your quartz. Kuro will be coming up soon as a welfare servant which at np 5 is close enough to Squirtle. Hans is just fine as a support and you can always use a friend's waver. You didn't mention best kouhai and I'll say right away level her. Mash is pretty good as is and becomes a monster post Camelot.
Also make sure to do the upcoming event and np 5 swimsuit shishou. You are a bit thin in the assassin ranks and she would be a step up from your CA Hassan.

Proto cu is a decent lancer. Herc is a 5* when fully developed though in a 4* body. Worth investing in. Nero is also actually quite good even if the difference between her being amazing and her being just decent is tied to rng of one ability. And Oppai pirate is worth investing in as well as she is synergistic in a lot of teams.

Tldr: invest in and ascend the pieces you have. Invest in Assassin Scathach to give yourself the missing assassin star gen in your group.

Herc alone can get you through everything up to Camelot so if your having trouble I'd recommend working on leveling your servants/skills rather than going after new ones who will still need to be leveled..

You can roll on the upcoming summer banners. Getting better servants won't automatically make content easier. Raise the servants you use to 3rd ascension at least. All of the servants you mentioned are very good. Try to get more free servants and make use of them. Save your sq for ishtar and miyamoto.

The aforementioned advice are good.
However I must warn you that if you are experiencing difficulty in London, the later singularities of America (boi 3x Cu Alter why) and Camelot (nuff said) will have you using those CS like bullets.
I advice you to slow down and farm for hands and skill mats and max ascend the decent ones.

I don't think there's a need for you to roll though. It is as they say. You need to lvl your servants first and what you don't have when fighting for a specific boss will be provided by your friends. Then you can save for the ones you like to get then lvl them and it's a glorious cycle.