The ideal CE for Saberlot

Knight’s Dignity. If you have it, of course.

Probably that MLB Hot spring. The increased crit strength benefits Lancelot the most, especially if he has another source of crit stars.

Imo, AE is better. You have a reliable source of stars via his skills but It does have a cool down. So during the down time, best use of it would be to spam your np which is where the art bonus comes in.

I would go for Another Ending personally. It boost his crits along with his Art NP. Though Hot spring is great if you have a Star Generator in your team.
If you can get it, 2030 is also great with him.

Another Ending is the ideal CE for Saberlot. He already makes his own stars and Arts damage is kinda his bread and butter

Black Grail is actually better than Another Ending/Hot Spring for Saberlot if you only consider overall damage. But if you mainly focus on crits, the 10% arts up would be more valuable than another 5% crit (assuming only springs is mlb) since his skills generate enough stars.