Who should I focus on next?

1. Mash + max skill all
2. David + max 2nd skill (evade)
3. Robin hood
4. Euryale
5. Gawain

first, you should try to level up 2 servant for each role, one aoe and one st. it is quite a common and boring reply, but is true.
i also suggest to level ushiwakamaru over your 4 star raider.
martha is a nice servant but is something to use during challange, and the pirate duo are quite gimmick to use.ushi np5 can outdamage ozy and quetz. Even now thet i have medb and quetz i still use her if.
robin can also fill your team. he is a solid archer, even for who have highter rarity option.
oh and burn gawein.... i hate him..... well dont if you want you can use him for farm.
but you can burn him to just so you know...

First I'd focus on Mash. Get her skills up to 10/10/10 as soon as you can. With the new Break Bar mechanic implemented, her defensive skills can help form the foundation for many team comps for going up against bosses with break bars.

Next, try to have an ST NP and an AoE NP for each class if possible. You already have some but I'll list them anyways.

Saber - Lancelot and Caesar. Gawain's base stats may be stronger than Caesar's but his skills are seriously lacking. His first skill's Buster Buff only works on certain maps and his Charisma is E rank. His third skill has three different effects which may not always be helpful all at the same time (10 crit stars also isn't a lot and his Guts only lasts one turn, which means you'll only use it if you can't stop an enemy from letting off an AoE NP). D'eon works somewhat well as a Saber Tank with his skill set but he isn't a priority. Bedivere is great too, especially if you can get him to NP5 (which may be hard since he can only be summoned in some banners and the story summon).

Lancer - Eli and Vlad are okay but you might want to consider Cu and Prototype Cu. Cu is one of the hardiest Servants in the game despite only being a 3*. Proto Cu may not be as tough but he does hit harder.

Archer - Arash is easily the best farmer for his class, good for taking down the first wave in story quests and even in many farming events. Robin Hood and Euryale are nukes (Robin Hood has better max damage but Euryale is a great anti-male and stall unit). David is one of the most popular choices for challenge quests because of his full party Heal and Evade.

Rider - Ushi is a great budget Rider Servant who can out damage even a 5* Rider due to her easy NP5. Georgios is one of the hardiest tanks in the game despite his low rarity. Alexander is one of two Servants currently in game who can grant Quick Card buffs to the party, the other is Atalante, and his charm, even if it only affects humanoid opponents, is not limited by gender. And despite his sleazy attitude, Edward Teach is a great support unit, especially for female servants (his third skill can heal the party for 2k, double for female servants).

Caster - Medea Lily is still the game's best healer (barring Merlin's spammable Heal per turn NP). Hans is the budget Waver, while Mozart and Shakespeare can grant the party a pretty powerful Arts Buff and Buster Buff respectively, even if it only lasts one turn. Medea's NP is pretty spammable and is great for removing enemy Buffs, including Guts.

Assassin - Cursed Arm Hassan is a great Crit star generator and, if you manage to get him later, Kotaro does a great job as well, though with a different niche. Sasaki Kojiro, after his rank-ups are done, is great for generating crit stars while also being able to use them for himself with his third skill. He's not the strongest, but he'll get the job done.

Berserker - Lu Bu is still one of the hardest hitting Servants though he won't be as valuable against enemies with Break Bars, not unless you can somehow keep him alive until his next burst. Asterios, again if you can keep him alive, is a powerful debuffer since his NP's Atk and Def debuff lasts SIX turns. Spartacus is a surprisingly hardy berserker since his first skill heals him for five turns, his third skill heals him for 2k max, and his NP heals him for a minimum of 3k. His HP is low, but he should do the job for the earlier singularities with a bit of help.

Some of the Servants I listed may not always do the job given their limitations (relatively low HP and ATK compared to higher rarities) but if used right and with the right friend support they'll be more than enough to get you through the game or until gacha decides to give you better options.

Hope this helps.