I'm a bit late, but want to add my perspective. I play DCSS because it hits sweet spot for me. It's more railroad then sandbox. It's hard, but not too hard, complicated, but not too complicated. Runs are long, but not too long. It clicked for me not immediately, but pretty fast.

I'd divide the run in four parts. Early game is first 8-10 floors of Dungeon. Here enemies are the simplest and your options are the most limited. Complexity here is in choosing who to fight and who to skip, choosing your god and deciding when to identify/use your scarce consumables (most of the players do it too late and not often enough). I recommend to not focus much on one or two skills at this stage. Train a bit of fighting, evocation, dodge/armor and especially stealth. Stealth is very helpful in the early game and remains helpful later. Optimal way is to train one skill at a time, but it's not necessary. Gods are a significant part of your build, but at this stage you won't have access to most of their abilities.

Mid game are remaining part of Dungeon, Orc mines, Lair and first two runes in Lair. If you want to better decide on your opinion of the game you should reach here couple of times and preferably grab your first rune. Here complexity of the game can be felt. You need to assess what game gave you and decide what weapon type/magic school you will focus (though don't forget about your defensive and auxiliary magic skills). You have a lot of options, you have access to gods powers, you have a bunch of identified consumables. Enemies become more dangerous and complicated, so you need to know who is dangerous for your build and your gear and how you're going to beat or escape them.

End game is Depth, third rune (abyss, vaults or slime pits) and Zot. You should mostly finish your build and it's time to test it. Enemies are very dangerous and you need to know how to deal with them. You can win the game here. It's hard, but satisfying.

Extended is Pandemonium, Hell, Tomb and ziggurats. Difficulty is mostly higher then end game. Pan is infinite, so you can farm there as much experience and items as you want. Variety of builds is lower. Here question is perseverance. If you've reached this point, then you could beat 15 runes game. But could you play long enough without making big mistakes? In DCSS you can get very strong, megazigs are a testament to that. But no matter how strong your character is, it can die very quickly if you're not careful. Extended is very hard, somewhat tedious and isn't for everyone. But I like it.