Fully paid off Mercedes suggests you're good with money and credit cards.

Learning about the nuances of APR suggests you might be learning the ropes.

If you ARE good with finances, consider opening a new CC to leverage the sign up bonus to charge the 6k repair fee too. It's not uncommon for higher end cards to give $500+ in sign on bonuses. Many will have an annual fee, just product change to a no fee card before it comes due.

If you aren't good with finances, disregard the above. It will lead to ruin. CC debt can snowball into taking years to pay off.

EDIT: Because you're explicitly asking if it's wise or unwise, imo get a second opinion. 6k for minor fixes is wild. Secondly, if you need to go into debt to maintain your vehicle, you do not own the vehicle, the vehicle owns you. I understand there may be personal reasons to drive that car, but if it breaks down on you and you can't afford to fix it, you may want to consider preemptively selling it while the going is good.

I would never max out credit. That’s bad for your credit score. And those tempting offers are great until you don’t pay it all off and get slammed with interest.

The dealership mechanic is always going to recommend $6K in maintenance. I would definitely not do this unless either it's time for them on the maintenance schedule in your owner's manual or you're having a problem.

Wise? Or unwise?

I think it's unwise, even though your bank account probably pays more, so you could make money on the spread. It's a hassle, a distraction, and a small but non-zero risk a payment won't go through or something and you'll end up paying fees or having to spend time getting them reversed. The after-tax earnings on the spread would be under $100 if you borrowed $10k on this card. Focus on doing the stuff that allowed you have a paid-off Mercedes and no debt.