I'm guessing the answer to this is "no", but I'll ask anyway. Is there an easy way to search the workshop for scenarios that don't require any DLC aside applinked from whatever route they are for?
From withing the game you can specify which DLCs are allowed during scenario search, you can also use a generic "All I own" condition.
I actually have a private collection I've been making for myself that has over 400 scenarios I've collected from the back of the Workshop. Send me a DM and tell me what routes you're looking for. I've been debating making a list of such scenarios. If it's a newer route since 2014, however, my list may not include them, but I think I have over a couple dozen older routes with scenarios not requiring other DLC besides the European asset pack (which comes with the 2022 batch of routes anyway, though U.S. doesn't). Note that DLC is always required, though. The route DLC is required in each case. Otherwise, you can't really have anything. I refer to the group as "single DLC scenarios". It includes Memories of Maerdy, which comes with a small route.
I’ve had to resort to using TS-tools to swap rolling stock in some cases if the scenario looks like it is worth the effort. In sometimes I’ve found the DLC is no longer available.