—Thor, Stargate SG-1, “Small Victories”
Maybe the UFOs were built to withstand alpha-gamma-laser-magma-pulse rays, not our primitive projectile based weaponry!
A premise I was skeptical with but utterly fell in love with once I read it.
[1] https://www.eyeofmidas.com/scifi/Turtledove_RoadNotTaken.pdf
Why FTL? or any of the things you list. We are already looking to fly tiny probes past alien exoplanets with Breakthrough Starshot.
If you can get balloons into earth safely, you can also just pelt the planet with big chunks of metal at speeds more deadly than nuclear explosions.
The air defense folks increased the sensitivity of radars and started finding the UFOs. Maybe the UFOs were keeping away from population areas and radars but now were being picked up.
It is strange that the objects aren't stealthed. Which makes sense for balloon payload. But stealth, hovering, car-sized objects should be hard to detect. Maybe aliens don't know about stealth. The other weird thing is that they just hover. For advanced or alien technology, I would expect them to move away or dodge.