It's surely not optimal, but then again selling buildings isn't really optimal, unless you are approaching the end of the game.
If however you have money trouble, you can safely delete amphitheaters form mid game forward. When your SP cost 500-1000 culture, I don't think losing 4-8 culture per turn would matter that much. If you have Sistine Chapel you probably have more to lose and things should be evaluated (but even so you only lose more than 1 CPT if the city produces a CPT which is a multiple of 4, because that 25% gets rounded down).
Regarding the slots for great works of writing, it is very rare you actually don't have a spot for the great Works of Writing, especially since you shouldn't be producing any yourself (unless you aim CV). Maybe produce one great work, if you get a great writer super early (if you have 20 CPT and get a great work that is a 10% increase so it is significant at the time, but fades pretty fast). Otherwise you should just keep them and bulb them when you have a decent CPT output to burn trough ideology or rationalism, etc. You may even save them to bulb during the WF bonus.
For cultural border growth you will need an additional source of culture (probably from religion), because otherwise it will be too slow and you still end up buying a lot of tiles.