I don't think you can drive to ATM yourself, you need to go with one of the approved tour companies for that. We used Pacz out of San Ignacio. Mayawalk is pretty high pressure and once they have you hooked they keep after you trying to get you to book other tours (according to other tourists we met).
If you want to enjoy Caracol and the trip through the Mountain Pine Ridge, you may want to hire a guide for that as well. We use Chelo who can be booked at Eva's (don't book with the other guy through Eva's!!) We saw the other guy stuffing a tourist in the back of his SUV and encountered him another day at the Benque border...he was scolding his group for taking to long to get through the line, like they had any choice as to how fast the person behind the counter was processing people!
I don't know if you would be allowed to take a rental into Guatemala to go to Tikal. We arranged to meet a van in Melchor who took 6 of us to Flores for a day for $100 US round trip....we had been to Tikal several times and decided a day in Flores would be nice. The gentleman's name is Edgar and we made arrangements through "Mates" who can be found at the taxi stand in front of Courts in San Ignacio. Edgar has a nice Toyota van with A/C and is very friendly. We took a taxi to the Benque border and walked across to Melchor.
Many travellers prefer to rent a vehicle, we prefer to use local companies if we are staying in one area as we enjoy gazing out the windows and we like to ask lots of questions of the locals. If you are travelling all around the country, or in several districts you may want to consider a car rental...do a search on this forum, some companies do not provide quick response in the case of breakdowns. Enjoy your trip!