Alright to start with. Good job on maxing out Mash, she’ll be your go to for challenge quests where you want to tank tons of damage and reduce it to nothing if you play your cards right. Not so useful with the unnerfed Kiara unfortunately. Kiara just weakens you so hard that you wish you were dead.
In any case, start combining your craft Essences with other congruent craft essences, eg. The Jeweled Sword of Zelretch with your other Jeweled Swords of Zelretch. If you get another two and enhance your original CE with it, you’ll Max Limit Break it which will max it’s potential level you can upgrade it, but it also enhances the CE’s effects on Servatns equipped with it. So you can imagine MLB’ing a 5* CE will be an enormous pain in the ass especially if it wasn’t event related. Even the 3*'s can be useful. I know the Battle of Camlann will give NP charge to the rest of the party when the Servant Holding it dies.
To start with get Single Target NP and AoE NP’s for every class of Servant, at least the first seven classes: Saber/Lancer/Archer/Rider/Caster/Assassin/Berserker. Look through what you have available, and don’t disregard 3* Servants as I’m sure your own Euryale will tell you from Camelot. At this point you’ll want to focus on this point the most. making sure you have an answer for every situation you could find yourself in. What i mentioned in this paragraph is just the basics.
There isn’t a single 3* archer that isn’t worth upgrading. Cu Chulainn (OG) is probably the best survivor with Heracles probably being the best. He’s well worth the investment.
Hans Anderson is your best bet for F2P Support unless you miraculously manage to roll Waver/Tamamo/Merlin or even Scathach Skadi next year. Also, Shakespeare and Amadeus are good for Buster buff and Art’s buff respectively.
That’s all i really have for you unless you’re looking for more specialized advice.