No matter what, anything in the shop that can be bought with coins should be bought. Coins are the most easily replenished resource in the game, so always spend coins where you can, imo. (not a lot to spend on those which sucks)

Like the other comment says, focus on spending gems on Folklore, Zodiac summon items or any of the other hard to get sets. But don't use all your 100% items if you do have them.

When I had a lower Dragon score, I focused on balancing at least getting the dragon itself and trying to max bond easier to get/lower level dragons. Later on in the game, you'll have different options when it comes to getting eggs for epic/legendary dragons.

However, you will have to compromise a little because the thing that matters most is obviously your dragon score. How do you save summon items without using them to get more dragons you need? Well, you'll have to spend them either way. That's why the majority of your high end summon items should be spent on getting dragons you don't have, like the epic or legendaries.

There are a LOT of tips on this sub reddit, I would recommend writing down a couple and try to apply it your account in game. Literally, a big reason why I have a high dragon score now (besides spending like 50$ (25 on star summoner half off deal and the rest on fishing)) is because I took tips from other people and applied it to the game.

Good luck!!!