Funny enough I’ve done just that, sold off a load of SNES and NES carts on eBay (I’ve had my Everdrives a long time now and they were just collecting dust on the shelf, time to let em go). Got myself a Jag system with A vs P and 3 other CIB titles to get the collection started, with change to spare. I’ve emulated the Jag in the past but nothing like the real system.
Yeah I would think twice before jumping down that Sega CD rabbit hole - those things are a good few years older than the PS1 and Saturn, many of them are scratched to hell and at a premium price. Touch wood I’ve never had a case of disk rot in my collection, but it is a real thing - one year after purchase you could find them unusable.
There are a lot of great Shmups on the Genesis - Fire Shark is a personal favourite and it’s not too hard, the Genesis version of Raiden is passable though if you have played Jag, PC engine or arcade version it probably won’t seem great. Truxton, Steel Empire and MUSHA are all worth a go too, particularly MUSHA, though not everyone likes it - and you’ll want to be using an Everdrive for that one ?