Easy way to keep up with multi-speed bikes on a single-speed is to make sure those you ride with are decades older than you, solved.
I always wonder why people have a hard time figuring out what ratio they need for a single-speed bike, don't they know what ratios they used to ride in when on their multi-speed road-bike ??? The gear ratio I used for almost all of my riding on my ten-speed Huffy road bike was a 52/19, so when I built up my "LeToaster" single-speed coaster-braked bike, I put a 52/20 on it and it is absolutely perfect, I nailed it first time. It is of course a bit slower on level road, but I was always able to go a bit faster than most I rode with so that is no problem, and the extra tooth on the back works fine for grades up to around 5%, if they get steeper than that I just get off and have a nice walk. I would never expect that there was some magic ratio that would let a single speed keep up with a multi-speed bike, nobody with any education would.