You've basically described what I have in my lawn and fields, except that I have blackberry instead of thistle.
What I use:
- For my lawn I use a self-propelled walk-behind Toro purchased from a big box store several years ago. The blade is in horrible shape (I should replace it), but I sharpen it with an angle grinder a few times a year and it keeps doing the job. This machine has been thoroughly abused: I knock down heavy weeds (small brush) with it, it has had numerous encounters with rocks, uneven ground, etc.
- Another tool that is very useful is a scythe. I use this in areas where the mower can't go, or there are just too many rocks and the ground is too uneven for the mower. Just be careful to mow high with the scythe or you'll ruin the blade on the rocks.
- I've used a string trimmer ("weed whacker") in the past, but sharp rocks tend to eat up string really fast, and they don't work well on thick, woody weeds (e.g. blackberry). I've seen some that have a blade instead of a string, but hitting rocks with a blade seems
- At a larger scale, I recently got a field mower attachment for my tractor. This is used on a couple acres of horse pasture. (Yes, up until this summer, I used the little Toro to knock down brambles in the pastures.) It easily handles small brush, and the blades will give way if they hit a rock. (Though I have already had to replace a shear pin when it hit a rock that was too big.)
- I have not used them, but I know people that have the DR brush mowers as mentioned in a comment on the question by @DA. You may want to consider something like this.
For any solution, it's helpful to:
- Remove as many rocks as possible from the area to be mowed. This avoids damaging equipment, and potentially throwing rocks that could injure people or damage property (e.g. your living room window).
- Mow high. Set the blades high to avoid hitting shallow ledge outcroppings that you simply cannot remove.
- Lower your expectations. With the conditions you've described, you're not going to have a golf course lawn. If that's what you want, you're going to need to have many tons of screened topsoil trucked in to cover the rock.
- Consider alternative landscaping. Some areas just don't need to be mowed. You may be better off deciding against a lawn in a problem area.
- Learn how to maintain the equipment yourself. At the very least, an investment in a vise and angle grinder, and time to learn how to sharpen a mower blade can pay off over the course of a summer.
