i know you asked the civperts and you know that i'm not answering as one. but i can give you my opinion as someone else who did go from warlord to noble; a lot of people on the boards didn't, they started out at noble.
i found noble to prince easier than warlord to noble. for me, even monarch to emperor was a little easier than warlord to noble, no kidding! partly because i had played a lot more by then, partly because i knew not to worry about watching the scoreboard, but i swear it's not because i wasn't a permanoob by the time i tried. it's because the AI behavior changes between warlord/noble in way that's hard for me to explain, but it's there. from my perspective, it wasn't
just that "at noble the human has no advantages but below noble it does". it's probably an indirect result of the discounts on maintenance etc, but it's not the tech rate or production. what felt different on noble than on warlord was that they settled land super-fast. OMG all of a sudden they were settling spots i thought i still had a bunch of time to get to, greedy dorks! getting used to that, and the tech difference / production difference that you see on all level jumps, was hard for me. now that you've played noble, you're used to them settling above the "we're reallllllllly slow, we're warlord and below" pace. i honestly didn't know how drastically that affected the game until i made that warlord ->noble jump, so if that was part of why that jump was hard for you, then this one won't be as bad imo.
they do spread out faster on prince than on noble, but not nearly as big as the difference is between noble and warlord. the only other level where the change in landgrab is
that dramatic is immortal to deity, and there it's of course because the sploiters start out with 2 settlers. note that i did my warlord -> noble jump on vanilla. i don't know how much it's changed in warlords/BtS if any. i sometimes play settler/chieftain/warlords games in warlords/BtS, it's wicked fun, and they still expand hella slow. i can't remember their expansion rate the last noble game i played to compare the rate to.
for me, noble to prince just wasn't as big of an adjustment. i mean it was hard at first to get used to, you still have to get used to the tech / production bonuses they get, so i had to re-think things and learn new tactics, but it wasn't harder than the warlords/noble jump. mileages vary of course, and we all know i'm a total freak anyway so the whole theory could apply only to me *giggle*. i definitely 100% think diplomacy gets more important the higher you go on levels, but i think you knew that already. it saves my life about 80 times each game so i'm biased

. when picking your friend(s), watch who their enemies are, because things can get ugly when they demand you cancel trades, blah blah. it's generally a bad idea to have two best friends who hate each other. kinda like real life that way.
do not pay attention to your score relative to the AIs, ever. while you're already paying zero attention, pay less attention the higher the difficulty. i know it's not possible to more than ignore it, but you get what i mean i hope. if you're in the middle or the bottom of the scoreboard, just pretend you're not on it, don't let it bother you. i'm serious, it really does not matter. it's based on stupid arbitrary stuff; the things that matter as far as winning the game aren't measured directly as part of the score, and can change drastically overnight. you can win from last place because you're smarter than them -- even your very first time on prince you're smarter than the AI, trust me! so i look at the scoreboard as a way to compare them to each other maybe, but i pretend i'm not on it.