The other significant issue is that you lose the proper mechanical advantage. So you'll find that the brakes which previously were easily applied and modulated well with only two fingers on the lever will now require a pretty Herculean degree of squeeze to stop very well.

Your goal for a nice older bike is to make up a casual round town and gravel trail bike.... So just stick on some suitable tires. A great option for that style are Kenda Krad tires. Smooth enough to be fast on roads yet with an open enough face to the block pattern to have a reasonable grip on packed gravel. If the Krad tires are no longer available then something similar. There's still a pretty good array of 26" tires out there.

In the distant past before disc brakes were common I looked at the idea of an MTB with 700C wheels. There was simply no way to make the pads line up without extending the adjustment slots on the V brake arms. And regular cantilever brakes did not even get that close to working out. The amount needed and the lack of feel meant that it was a non starter. I had to wait for disc brakes to become a touch more mainstream.

I ended up converting the fairly high end Stumpjumper shown below to 700c with a disc hub and 700C rim build and used it for sloppy weather commuting for about 6 years until I retired. If you can see what this is you know it wasn't a cheap option either. The VERY next year Kona introduced their disc version of their Dr Dew bike at less than half the price.......