In addition to what's been mentioned, the thickness of the rim strip can make it much harder to remove and install a tire. You want to get the tire beads to be down deep in the center of the rim except for the one part of the bead that you're prying over the rim's edge. A thick rim strip keeps the bead from getting down as deep in the rim. Try using a thinner strip or even Veloplugs which are probably the best option albeit also costly.

I haven't noticed any particular issue with steel beads compared to Kevlar. Steel-beaded tires generally seem a little easier to install when new, but the Kevlar ones get easier over time since they have a bit of stretch. But the main problem is that some rims are just a bit bigger diameter than most and some tires are just a bit smaller - combine the two and it becomes much harder to get the tire on the rim.