Burning CEs

Been playing since day one and have not burn any CE for mana prism. Reason being is CE can only be gathered reliably though friend point gacha. The 3 stars or lower are used for levelling up the CE to give more attack or hp to your servants, plus added benefits and even more so if it's limit break. Mana prism can be easily acquired by going though events, daily quest (30ap ember if really need to farm mana prism) and burning duplicate servants considering the 6th copy of a servants will be of no use other than taking up space in your second archive.
For which CE to keep and which to user for levelling up other CE, I personally keep all my 4 & 5 stars CE, keep event 3 star CE (only if the rerun is coming back), and merge for 3-1 CE for CE bombing. The only time I would consider burning a 4-5 star CE if an event already had a rerun and can no longer be acquired to limit break again, but would still avoid that considering 3-1 star CE bombing is just as viable.

I always keep at least one copy of any CE just for collection purposes. If I have 5 of them, I’ll usually just MLB it. Other than that, I sell all the friend point CE (but keep one of each).