Didn't they devolve into RTS ?
Yes, but I'm not that sure for the Settlers II Remake, and for the Settlers 6 I can't say that much as well - only played the demo.
But for the earlier Settlers, maybe.
Cultures would be a good game imho as well - It has definitively a Settlers feeling in/on it, but plays a little bit more different ... The enhancements are in the details, and it isn't that RTS-like than the later Settlers. Fans call it "the better Settlers".
In its style it is like the Settlers until 5. The Settlers 5 were quite a break from the former games, much, much more RTS, and a different art style.
When I played The Settlers 5 parallel to Spellforce, at one point I didn't find any difference anymore between both in terms of game mechanics. That depressed me, and I uninstalled both immedeately. Because I'm rather a fan of the gameplay of the Settlers 1-2 - and of course of the Cultures.
To my shame I cannot say a single thing about the Settlers 4, since I've never played them.
Settlers 3 is already an departure from the Settlers 2, already into the direction of RTS. This results in the absolutely horrible thing that in solo play you always have the writing "You have won! above the screen, because the developers seemingly developed The Settlers 3 never with real soo play in mind. Atr least that's my very personal opinion.
Playing The Settlers 3 in solo mode alwys with that writing "You have won !" above just sucks. To me. And I haven't found any way to make it go away. And I never understood why no-one was complaining of that either.