TLDR: if you want to do sand and mud but keep road performance I would get the lowest RR knobby tire you can find and keep it reasonably narrow to limit the aerodynamic penalty. Something like the Continental Terra Speed in 35mm, if you can make that work comfort wise.

Anyway, one of my quarantine projects has been trying to figure out how to "test" different tires and tire widths on gravel. And it's basically been a shitshow of meaningless data, but there were some unexpected observations. I believe the following at this point:

• On the road (@~20mph) a Vittoria Terreno zero 35mm 700c slick is faster than the same tire in 47mm 650b, as expected
• On gravel, I have no idea regarding the slicks. They seem about equal @~15mph, and at @~20mph maybe a small difference in favor of the 35mm. Nothing in the data here as it's all inconclusive.
• On the road (@~15mph) a 40mm Continental Terra Speed isn't that different than a 2.2" Continental Race King Protection. @~20mph it's pretty substantive (double digits watts) and I can't sustain the power needed to maintain @~25mph on a few mile flat section with the Race Kings
• Both of the Continental tires feel fast on gravel, and again the data on power vs. speed is totally useless (and they subjectively feel just as fast 17-20mph), but I happen to have tyrewiz sensors on my both wheelsets and on gravel after a few miles the PSI on the Race Kings drops by 10%-15% vs. where they were when I got onto the gravel and will go back up when I get back on tarmac. The change on the Terra Speed is 2%-3%. I am going to continue to watch it I think it means that the Race Kings are losing substancially more energy due to casing deflection on pavement than they do on gravel due to the combination of knobs on a hard surface, having a greater contact patch, and running a much lower pressure.

My personal experience is also that knobs matter more than width when going through short runs of mud, but don't feel as good on regular gravel. Also, no gravel tire I tried comes close to a GP5000 TL road tire.