“the region between my knees and hips can get chilly.”
Some Nordic ski tights have a wind-proof panel on the front of the thighs to add some more wind resistance there (especially relevant for racers since their own motion creates wind on the front).
But since you’re looking for something you can add and subtract without “dropping trou” how about two swatches of fleece or wind-blocker fleece cut from a thrift-store garment? About 10″ x 13″ each, you could slide them between the front of your thighs and your tights. And even more easily pull them out when you get too warm.
In a pinch or as a proof of concept, you could stuff any number of items under the tights like a beanie cap, extra socks, bandana, etc – items you might already have with you. If you aren’t concerned about the outline of a sock under your tights, that would be the UL thing to do – multipurpose more of your gear. A (poor) grad student I knew at Berkeley would buff up his light shell for a cold bike ride home by asking for a newspaper, crumbling the individual pages, and stuffing them into his shell, especially in the front.