Exactly how hard would it be to place a “cork in the hole” to render the cavity unusable? If (big if) overpopulation becomes a problem, it’s pretty easy - these days - to develop and maintain a database of most of these swift cavities, survey them from a distance to see if they are corked or not, and adjust the number of corks as appropriate to address current population trends.

I get that you don’t like the approach - but it’s a solid one, which is what works best for swifts’ nests: solid structures.