This and your other statements feel like you are looking for excuses to exceed about every limit you need to adhere to.
If you search the forum you will notice others have flown balloons as well. Adhering to the maximum transmission power and using SF10 they reached over 600km and hundreds of gateways. May-be you need to do more research and try to adhere to (legal and other) limits.
Check this experiment on ttnmapper for an example.
BTW, SF12 on moving objects is a bad idea in itself. Check this post.