Best way to spend your crystals?

Refreshes. And sorry, I'm too jaded by the game and the questions been asked too many times to give you a long elegant answer.

Suffice it to say you can get a number of rewards from refreshing energy and farming (and not just the one specific thing you are farming) such as currency, mats, credits, other gear, etc that all add up to pushing you forward.

Then when you have a developed roster? Refreshes are still better.

If you are under 85 I think you should spend all your crystals on energy refreshes, since that also gives you xp. Keep in mind that farming gear is generally more important than stars. Once you reach level 85 then it can be more efficient to save your crystals for buying gear, but it is best to focus on the gears that can't be farmed from nodes.

Gear

So after 70ish I've to count if it's better to buy an item instead of farming it for energy refreshes?

If you're not going for a team like Nightsisters where some characters are only available in shipments, and if you have crystals left over after your refreshes, hoard them. There will come a day when those completed Mk V furnaces for 1400 crystals are money well spent.

There's gear you can only get from raids or by buying it, and raids don't drop it nearly often enough. Crystals will help with that a lot.

If you are doing all three energy refreshes of both types of energy (always these first), then gear is not a bad idea. It can depend on your situation, would a 50 refresh in arena or fleet arena help you climb higher during payout to earn more crystals? Do you have a bunch of credits but need better mods, so a few 15 refreshes for the mod shop looking for good mods could be useful? Is the piece of gear you are looking at a long farm or is it a piece needed for your arena team/fleet pilots or TB/raid toons? Once you know why you want something, it makes the decision easy. And if you do not see any that seems like a good deal and none of those other options sound good right now, hoarding crystals is also fine.

Imo, u have to set ur minimal crystal limits. So when there is "panic farming" event u won't caught off guard. Also for backup if there is event u want to refresh bcause the reward u think is good. After u surpass ur minimal crystal limit, u are free to buy whatever u want.

I have mine set at 10,000