Should I spend my embers?

Max out scathach then save the rest imo

You should really max out Ruler Jeanne. Once she gets her upgrade, she becomes really good, so I think that's better long-term investment, especially with EXP all embers.

Scathach Lancer is nice to max.. I feel Fran (Big bang glass cannon) and Karna is pretty good too.. for Farming eg daily ember farming.. save the saber embers for Elizabeth saber when she is out later.. since she gives bonus dmg up and pretty decent unit..

I feel bad hearing that, hope you're lucky next time.
Maybe my explanation here can help you a bit.
Every SSR is worth raising, Scathach can bring massive Burst damage in a few turn.
She and Karna hits real hard, i would say they're your best DPS alternative for replacing Jalter.
Joan Of Arc is a great party Supp and Tank, she'll be upgraded soon.
Tamamo Summer is also a Solid servant, for longer fight she is a good ST lancer to pick and good for slaying male bosses.
Karna is an AOE but his damage is very high, can be used as a Buster Crit team Main DPS.
His main job is farming and wave clearing, which you use everyday.

It's just a little example, i would recommend for completing the Class Triangle first.
And of course raise them based on the hole on your team a.k.a your weaknesses.

Though, the latter is up to your choice.
Best of luck.