This is where I really want to tell you to just go on a horse and learn to post while trotting because doing a posting trot is basically squats, on a horse, at a moderate pace. The only difference is that while riding you don't feel like you are working hard on flexibility, mobility and strength along the entire leg. This is why I can post for 30 minutes (which is easily over 100 squats) but I can't do 100 squats on the ground because the ground puts a lot of stress on the joints and I have cranky knees.
However, you may want to check if you are doing your hips and ankles a favor by seeing if you have all of your limbs moving at their best. When I do squatting at home (because yes, I do work on it at home), I make sure to have my feet in the right position because having your feet not in the right place will limit the knees (and knees being cranky means hips will hurt and that's just not fun, right?) This is a great article that explains what you may be doing wrong on your ground ("normal") squats. If you are still finding yourself tight, hip openers!