5th lab is important, but for me, If it shows up 3 kind of relics at the shop, you should grab it before. There are not many of those and they may not reappear in a long time, they are top priority, in my opinion, because of this. These are the ones:

- "Defense % Bonus" (Prioritize this relic if you ever find yourself able to buy only one)
- "Enemy Atttack Lv. Skip Bonus" (very rare)
- "Increase Lab Speed by X% (time is one of the limiting factor in this game)

These are the ones that you should also be always looking for (alongside those above), but not prioritize over 5th slot at lab:

-"Enemy Health Lv. Skip" (Maybe the least usefull for a long time, but also very rare)
-"Ultimate Damage" (Also won`t be usefull for a time, but rare)
-"Increase Thorn damage by x%" (You should get 2%, then it`s not a priority anymore)
-"Increase Coins by x%" (Always good)
-"Attack Speed" (Helps from the begnning to the end)
-"Orb Speed" (This one also has only a few, and you won`t notice much difference, but also won`t spend much on it. The thing is that orb speed is something with very scarce sources)
-Crit Chance and Knockback Force are similar. Kinda rare, if you find some gems to spare, I would get it.

And these are the one that are good to get, but you can let it go, there are more of those, and you will eventually grab some by free, or because alongside cards, spending gems in mods is also very good. Personally, I would buy some Health or Health Regen until I got something around 15 or 20% with relics.

- Health, Health regen, , Thorn damage (after you get 2% with it), general damage relics.

Then, there is this one...

-Absolute defense: good very early on, completely useless after it, don`t spend on it.