USAF, RAF and others are making daily flights by E-8 JOINT-STARS, RC-135 Rivet-Joint, etc, along the Moldavia, Ukraine and Belarus borders, in order to conduct SIGINT and monitoring of Russian forces across hundreds of kilometers, so the 18 km altitude of a balloon is hardly going to be a problem.
And if you can provoke a hostile fighter to actually use its targeting radar against your balloon's SIGINT suite, that's a bonus.