Well, neither Holon Fossil or Strange Cave help get Rampardos into play anyway (unless those cards were going to be errata'd to include all fossil evolutions), and as you said, Strange Cave is rotating out. The problem you mentioned about SSUing a "stage skipped" fossil evolution and then trying to get it back in play without the fossil itself is actually not that big of a deal in my opinion anyway, since with Paleontologist you can easily get the fossil either from your discard or deck, plus there is the chance that you had already drawn into the fossil before and had it just sitting there in your hand. Either way, you're able to get that evolution back into play. Also, if you still had Strange Cave on the field, you could just skip the stage again with it. Or you could play another Holon Fossil and hope for tails (which feels so strange to type...).
I also think that fossils are not as bad as you make them out to be. Rampardos's fossil is not exactly defenseless since whenever it is damaged, you have the possibility of doing unlimited damage (I know most of the time you won't hit them for more than 20 if anything, but still). 50 HP is average for Basics. Also, you have the option of discarding it from play whenever you want to, which can save it from a quick KO. For instance, if you were forced to start with it and they went first, on your turn you could draw/search into a legitimate Basic like Riolu, ditch the fossil and bring Riolu active (and maybe even Candy into Lucario and start applying the beatdown), then later on, simply use Paleontologist to bring the fossil back into play for Rampardos' sake.
The whole SSU idea isn't even that essential to the Rampardos/Lucario idea I had. It just seemed to me that it, combined with Buffer Piece, would be a good way to keep Rampardos around for repeated Hasty Headbutts. The main idea is to use Lucario's Aura Sphere to get everything on their bench within KO range for Rampardos, either through its Hasty Headbutt or Assurance. Factor in disruption cards like Cessation Crystal as well as the general speed of both Lucario and Rampardos (the latter with the help of Paleontologist/Candy and perhaps DRE, of course) and I still think it could work well.