Renew video surveillance – current status?

What would speak against HD-SDI?

Nothing at all speaks against it! SDI is great and Hikvision has very good SDI cameras. Just recently had it in a project that also had a lot of coax in concrete. I use coax or 12G-SDI, for example, in home theaters for preview monitors or longer distances. But the cables must be in good condition; if necessary, I would borrow something from Amazon and test it.

I would recommend Unify, now with facial recognition, AI-supported search, etc. But they run on POE.

Maybe take a look at the wireless solutions from Eufy with solar?