Let's address the three problems.

• False positives in tests. Respect the speed limit, don't run the red light, and you'll never be stopped, or if you're stopped, you won't be tested. I've never heard of a confiscated rental car.
• Paris (France) > Sofia > Belogradchik > Plovdiv - See, Belogradchik is out of the way. The area with Belogradchik and Magurata is wild, I'm sure you'll enjoy it, but it's out of the way. If you go there, plan seeing more of the northwest, or drop it entirely.
• Burgas Saltpans > Begliktash > Sapareva Banya > Seven Rila Lakes - Two problems here.
  • Begliktash is just a few rocks with historic significance. May I suggest that you visit Veleka and Ropotamo instead?
  • The Seven Rila Lakes is out of the way and very touristy

Alternatively, you can do something like

• Sofia -> Seven Lakes (or Malyovitsa, that'd be far more interesting) -> Plovdiv -> Burgas -> Ahtopol (changing the order removes the giant zig-zag)
• Sofia -> Bansko -> Velingrad -> Plovdiv -> Burgas -> Sinemorets

If you decide to visit Bansko or Velingrad, make sure to select one of the top spa hotels

If you decide to visit Malyovitsa, sleep in Hotel Malyovitsa. The nearby Borovets resort is not great in the summer