I've merged your post with a couple of other duplicate threads and had a quick look through them too. Depending on where you are in the game will dictate gem spending. There is some excellent advice in this (older) thread which is still totally valid.

Most would advocate alicorns/energy givers for gem spending though. The good news is that there are now ways to get items through events, Mythic Research Centre, weekly doorbuster sales as well as Black Friday, Christmas and other sales offered during the year.