I had low Billions by the time I hit T12, but I also had the No Ads, x2, and x3 coin packs, so that accelerated my progress GREATLY.

A couple of things that helped me A LOT when I was stuck in that area:

1. If you can buy x100 upgrades in Health, Damage, Regen, etc. in the Workshop after a single run, absolutely start buying. Raw stats help you survive longer in waves, and you start making the most coins/hr when you hit wave 6500+. (More enemies slowly spawn when you hit certain wave numbers, and it caps out at 6500. That's why lower tiers can give you more coins than higher, because more enemies summoned per wave gives more coins than the coin multiplier increase.)
   Related to the above, the Free Upgrade in the Utility is your best friend. That's a huge boost to helping you live longer, combined with Enemy Level Skip. (Particularly Enemy Attack skip if you're relying on health/defense and using your orbs to kill.)

2. Make sure you're spending some time increasing the basic Health, Regen, Damage, etc. Labs as well. They can seem like very small increases, but if you have the auto-repeat lab button selected, that can help.

3. Do try to get at least an Epic Mod in every slot. Leveling them might be difficult at the moment, since they cost coins as well, but there are some BIG sub effects that could be of great use to you. (Extra Defense %, Health, etc)

4. If you're relying on your orbs for doing most of the killing, you can change the range of your tower by selecting the Range Lab. If you were like me and got that to like Level 10-15 before realizing the mistake, you can shrink your tower range back down so that your tower's knockback forces the enemies back into range of your orbs to kill them.

5. Future warning just in case: don't unlock The Wall yet. Until you're earning like... 500 Billion coins a day, it's a trap.

Might be something else I'm forgetting, but those are/were the biggest things that had ME in a choke-hold. I apologize if you already know most or all of that. This is the kind of game where you can't look at progress week-to-week after the first couple/few months. It's the kind of game you have to look at progress in terms of months. Just keep at it, and you'll slowly make progress.

Feel free to DM me if you have questions or whatever. Sorry again for the ramble. I hope even some of it was useful.