When you're doing solo kick/punch drills and lack a bag or a target of some sort to give your impacts some resistance, your only choice is to practice the drills with a bit of acting and imagination. It's less about conditioning your body than it is about training your mind / body connection. It's about getting those combinations drilled to the point where you can use them without thinking about it.
You can stop yourself in the air in the same position that your leg would naturally be in after kicking a bag. It won't feel the same, of course, but for what you're actually training, it's okay. All you're doing is working on the right sequence of movements. You can do that with your imagination.
Now the worry is, if you do a proper Thai round kick, it will cause your round kick to go through the imaginary target, which will result in you having to reset your body position after that kick lands on the ground where it's not supposed to be. So to fix that, you're not going to let it go past that imaginary target. It's an imaginary target, so imagine your leg being stopped by it.
If you're worried that's going to change the way you do your kick, well it will. At least for this drill, it will to some degree. It will mean you have to start the kick the same way, but you'll have to slow it down in the air as it hits that imaginary target.
This is not a problem. You're still not doing the "slap" style kick of, say, Taekwondo. You're still performing the kick the proper way in Muay Thai. It's just that you're slowing it down in the air as you imagine it hitting that target.
The purpose of this kind of training is to train your mind body connection. It will drill those combinations. It's fine, but it's not done for conditioning. If you want conditioning also, you need a heavy bag.
Incidentally, there are options for when you need a bag to use for conditioning but don't have access to one. You can try open gyms which typically cost $10 to use the bags, weights, and other equipment. You can rig your own bag at home if it's possible to do without annoying your neighbors (no apartments). You can also try wrapping trees with mats and bungee cord. Or get a friend to come over and hold one of those foam body shields.
Hope that helps.