Game got updated. Grand Merchent did bid 30000 plain after some time. I had about 24k.

Confirmed that a New Game+ is a hard ceiling from the Merchant of 30,0001. But it's pretty easy to earn this by manning the shop for about 20 minutes. Even got Bat a few more levels and coin from her.

I had 16000 loan. I removed it quite quickly without a huge increase. Here's how.
What I did is I would go to Tutnik's shop to sell things there for a while, until I get 2-4k. Then I'd go back into Enterpriston and spend all this money on loan and taxes. Then I'd go back to Tutnik and repeat.

The most important thing is : Your debt doesn't increase if don't meet a tax collector. Only your taxes are. So you can "stop the clock" so to speak.

Do note that the cost is higher for a challenge mode game (30 001 coins if I remember correctly).
Can't tell you which challenge though, had all four active.

I checked it, and yes, after you quit the tax bird, the tax collection starts to count.

I didn't knew the selling items prior to the auction were still with the tax collection stuff.

Technically the more you sell, the higher the tax is for the first time round in the store are Enterpriston right? So you will need to earn more to not get into tax issues.