What's to come

I don't know what servants you have, but I can tell you what's coming up so you can decide for yourself. The closest event is going to be the Moon Goddess re-run, which one of the 40AP free quests drops hearts. Pretty easy node, 84k demon on wave one, and wave two has 2 sabers at 50k and 70k, with a 30k berserker. Personally, due to how rare hearts are, I'm going to be farming that.

Then there's a Dantes rerun immediately after that. Euryale is a good pick to strengthen because of her charm.

The next event after that is Valentines, which is complicated.

I know it wasn't much advice, but there's some servants which are never a bad idea to work on. I'd say the list includes: Caesar, Euryale, Robin Hood, Arash, Lancer Cu. I'm sure someone else can explain better than me, but at least it's a start.

The only one I can think of is the nerofest for next year which has another lottery, but that's in September.