I'm just gonna answer some of the questions here...
If we are going with the general/classic "blessed by multiple Gods" route ala Perez and Rucka, then Diana's blessings don't really have a limit. I'm just going to focus on strength speed and durability since that's usually the focus and yeah, I expect Nubia to be on par with this.
In terms of strength and durability, we see that Demeter granted her with strength and power "like that of the earth itself". We can assume that this was year one base Diana, since under Perez she slow grew into that strength. However, she's also been stated to be Stronger than Hercules. Hercules is exactly as strong as Shazam, since he gave Billy his strength, meaning we can essentially use both their feats as one character. Hercules himself has done the whole "lifting the infinite weight of the Heavens" thing like in myth and Shazam has punched black holes into existence on top of fighting Superman several times, Diana has bested both Hercules and Shazam in one-on-one fights to prove the Stronger than title, I believe she once fought Shazam but it was a tie way back when. Diana herself has stated that she's never found a limit to her strength. In short, Diana is stated to be stronger than Hercules, a guy that has lifted infinity, Diana being stronger is basically a fancy way of saying "she has no limit".
Speed is a little more complicated. Hermes grants her just speed (we don't know how much) and flight. It's clearly beyond human comprehension. She's blocked millions upon millions of faster than light god shards, kept pace with Flash at over 9000 mph and could easily double it, has moved way faster than light, and has both stated and proved to be "faster than Superman" via Batman and their fights. She's also been said to be swifter than Hermes. However, Hermes has been shown to (at least in the past) run circles around flash. I don't believe for a second that Diana is faster than Flash. So, IMO, it all depends on what definition people want to use for swiftness. I consider swiftness to be more about combat speed then raw point A to point B speed. This is why I separate raw speed and combat speed/reflexes. Does Diana have a limit for speed? well, yes and no. She should easily be one of the swiftest fighters, capable of fighting at flash level speeds, which in on itself is absurd, easily faster than light. However, she does have a limit in raw speed, which is basically several steps below where Flash is, the same rule applies for Superman in this regard.
So... yes, her powers are mostly limitless. None are given an exact level. Really, only Athena and Aphrodite could even be considered granting Diana access to their "full power", Wise as Athena and Beautiful as Aphrodite. And even then, on paper Athena just gave her general wisdom, so again no real limit. It's hard to measure given its limitless, but it definitely shouldn't stop below city level. Outside of speed being really measured with Flash like every other character, she normally is put up against Superman, Shazam, Thor etc...
Again, I
want/expect Nubia to be at this level.
However, I agree with BitetheBullet. Nubia gaining power in Dooms Doorway comes with more questions than answers. If Nubia started out as a regular amazon and gained Diana levels of power via proximity inside the Doorway, why don't ALL the amazons do that? Why not just move Themyscira into the Doorway via magic and just stay there to get more powerful? how does this affect or not affect the demons already in Dooms Doorway? This is just off the top of my head. Dooms Doorway was originally just a mega prison that connected to the underworld. Why make it more complicated by adding this mystic energy thing?
Nubia becomes Champion of Dooms Doorway, as such she is blessed by the patrons with the same power and strength as Diana to guard it effectively. There, not complicated, everyone gets what they want.