I also agree with Tom,
I’ve been running Synology Surveillance Station for the past 5 years now, and i’ve found their generic ONVIF driver works excellent for most cameras.
Any vendor who drops support for that is going to be shooting themselves in the foot, since it’s a clear direction that they don;t want you using their cameras with 3rd party systems like synology.
My advice would be to download the latest firmware which supports the ONVIF and keep that somewhere safe for later on. Then make sure all your cameras are on their own separate network which cannot access the internet but can access the Synology NAS. Tom mentioned this in one of his videos for cameras which are no longer supported and therefore have outdated firmware etc. That way if a vulnerability is found in the firmware on the cameras then they are isolated the the risk is minimal. If you buy any more cameras, then you should be able to load the firmware onto them that you saved, but I would imagine if any new models come out then they would likely come with a more up to date firmware on them and you may run into trouble trying to downgrade the firmware.