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I do think it might not be too bad for you that SEB Lems isn't available for download at the moment, because as great as it is, that's precisely because it's not an easy pack at all, and definitely not one I'd recommend starting with. Still, if you want to try it, I still have it on my PC, so just shoot me a PM and I can send it to you
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There's a couple of "easy-but-not-total-joke"-packs, some of them are only available for Old Formats NeoLemmix, though, like nin10doadict's CasuaLemmings, or, one of my personal favourites, Colorful Arty's SubLems, which combines very pretty-looking levels with a steady and lenient difficulty curve.
Nessy's Lemmings Migration is a little harder a nut to crack, but it's comparatively short, so that still gives you quite a feeling of progression even if you take a little longer to solve an individual level.
There are also a couple of really easy packs, e.g. by Yung Gotenks. There isn't much in the "middle", though; most longer packs, like SEB Lems, Lemmings Reunion, Lemmings United, Clammings, PimoLems, NepsterLems etc., are also quite hard at the same time.
I hope my own flagship pack Lemmings World Tour falls somewhere in the middle and therefore fills that "vacancy" - but it's also the longest pack here on the forums so far
. And I have no idea yet how difficult it actually gets towards the end. Plus, it's also still in Old Formats; a conversion is planned for whenever Arty finishes his Let's Play of it, so that I don't have to update all the levels twice.
Overall, SubLems and Lemmings Migration would be my main recommendations. SubLems is totally worth installing Old Formats for it, in case you don't have it yet!
You can simply keep a separate folder for that next to your main, New Formats Lemmings folder.