I suggest as wide as will fit - unless you expect to ride on the very smoothest of roads. I've been riding a 26" tire equipped mountain bike that is setup up for road riding with 1.5" slicks. I ride with people who have all kinds of bikes - some quite modern and pricey. I can't notice any difference in coasting when I'm riding with them. If I'm drafting on a long downhill and coasting, I have to touch brakes. If there's a difference in rolling resistance, you'd have to measure for it. I can't notice a benefit otherwise. I'm running these tubeless. So my take is to go with comfort when there's no real gain in performance. Maybe consider running them tubeless. Heck, even if there were a minor penalty in performance I'd still go for comfort. I'm also 65 and am less tolerant of unnecessary suffering these days.

I'm running 28c front and 32c rear on my 1984 Trek 770. The 32c on the rear is pretty sketchy. I'll probably try 30c front and back. There is that middle option.

BTW, actual widths on the bike will vary depending on rim widths and tire brands.