Take a look at Eufy. My use case is likely different than yours, in that I rely on the Apple HomeKit Secure Video (might have the product name slightly wrong), which Eufy supports. So the video does go to iCloud, but AFAICT Eufy never sees it. And without going to Eufy's web page, I'm pretty sure they don't care to see it, as they seem to make it a point of "no monthly fees, no cloud" or summat.

Anyway, check it out for yourself, see if it fits your parameters. The one thing I'll caution is that their battery estimates are outright lies if you access the camera on a regular basis. I'm about to run USB power to the two cameras that get hit the most, just so I don't have to recharge once/month (as opposed to Eufy's estimate of twice a year.)

Signed,

A Dude Who has Gone Through Way Too Many Security Cameras

I had a look to cameras supported by Synology, but either functionality was behind a paywall or simply not available (stuff like movement alerts).

I settled for a TP-link.