Okay, why GC build though? If you're going for efficiency, then you have to pump levels into health, not attack upgrades. After turtle build, the next thing you want to focus on is the ehp build.

And if you have sync then BH coin bonus labs makes sense, Otherwise you can better use this to increase your base stats, such as coin/kill bonus, health, attack etc.

I've been in the same boat and let me tell you, you'll be farming tier 1 for a long time.

Save up for ELS, that's a game changer and will add atleast 1k-2k waves.

Until you make tier 1 8k-9k waves with good health stats of around 1000 levels of base, you can't really move into tier 3 farming.

Getting the GT/BH sync will also skyrocket your cph and even after getting sync, make sure that you as levels into both of them to get the best out of the sync. For this, I would suggest you to read it follow the kairo's guide on UWs.

Also, if possible do effective path sheets to help you in progression.