Ishtar or Scathach?

I'm my opinion, of those 3 options, taking into account your current ones, I would say Musashi. She's too good of a saber.

You're very well served on Lancers and Archers, try rolling for Musashi.

You lack options on the Saber class but i wouldn't say that you don't have strong Sabers. Brave Liz is fine as long as you don't press her stupid button and Siegfried is a work in progress...he will be one of the greatest 4* Sabers later.

If you want an outrageous Saber for gameplay Bride, Okita or Munemori are the strongest options. Musashi is also a good option if you like her.

Unless you want Okita, I'd have to say Musashi as well, since saber is your weakest lineup out of the three.

Ishtar - Useless goddess (according to the community)
Scathach - God Slaying Queen, seriously, she nukes Gil so easily.
Mushashi - Heard she's tricky to use, but if you pay attention she's very powerful, but also waifu (after Scathach).

Honestly, if you're not rolling for Waifus save for Mushashi. Siegfried is going to be an amazing Saber but it's always good to have another option.

P.S. Eresh - Wonderful Goth Goddess

Well i'm pretty objective about ''good'' sabers there.
They're good enough from gameplay point, maybe the word ''good'' from your point of view is your favorite saber.

Liz is a welfare 4 star NP 5 saber that can slay most bosses with ease. Her only drawback is her 3rd skill and it's not a big problem because every servants have weaknessess.
She can do well in a Buster team, though it's still up to you how do you use her so you tell Liz isn't good enough as a saber.
Maybe something is wrong on your team building/saber usage.
I think Musashi here is your favorite servant, just roll for her.
Or you just only think the best saber on gameplay is the only best sabers available.

Siegfried has been buffed and will get another buff soon. You can try bring Georgios, turn everything into dragon and his NP damage equal to most 5 star servants and pretty much exceed 5 star ST Saber in damage alone.
He is good at farming, AOE wave clearing, George and Siegfried for general boss killer.
DPS, Supp and Tank is a Classic balanced team comps.
He is good enough from gameplay perspective,

Pretty much, you've covered ST and AOE roles in Archer and Saber class so that's enough.
You have hole in your Lancer, you don't have AOE ones.
Saber - enough, another additions is for collection or waifu/husbando purposes.
Archer - enough, another additions is for collection or waifu/husbando purposes.
Lancer - ST roles, enough you need AOE ones.

Good sabers exist from the beginning of FGO - Faber, Fergus arrived after some updates.
I don't have any difficulty using him to slay bosses like Arthuria Lancer or even Nerofest Challenges using lowstar sabers.
And the only servant class with their lowest rarity '' R '' or Silver.
FGO can be cleared by using 1-3 star servant, so 1-3 star sabers isn't ''good'' then.
Then why FGO can be cleared by using them when they're not ''good'' enough.
Fergus can solo Goddess Rhongomyniad and some strong lancer bosses.
Most people without Gold sabers choose Faber/Bedivere as their main and no problem with that.
If only one 3 star can defeat the Bossess alone, what if they got any help with their fellow lowstars? FGO is beaten that is.

If you really want Musashi as your waifus then go ahead and good saber always exist.

Bonus, notable Sabers :
- Rama, Lancelot, Nero Bride, Salter, OG Nero, Attila, Okita Souji, D'eon.