There are only 5 non-combat skills yes. "Maxing out" is relative, as there isn't a max value from what I now. The theoretical maximum is 2 bonus from background 2 bonus by class, so 4 bonus points and with the 66 points you get to 11, so 15 total. But the question is whether you will ever need these 15.
But same as Abilities besides of dialogues there are no breakpoints.
What I totally hated in wasteland for example was that only an int value of 3, 6 and 9 (or some other values) made any sense as it brought an additional skillpoint per level. Same for the action point system. So if you chose anything in between your character is gimped. No way to make it sound better.
Thats not the case in Pillars. All the Skills and all the Abilities are "floating" and each point gives exactly the same bonus as the previous one.
With mechanics for example, which is also used for lockpicking you can also open locks if you do not have the apropriate skill. But you will need less lockpicks.
Well, lore might actually have a break point as you need a certain lore to cast scrolls from spells. Athletics makes your character getting tired less fast. Stealth makes you sneak better and survival increases efficiency of potions. But you might also want to increase your athletics on all your characters by a couple of points. Because if you are climbing up some broken stairs, a character with less than a certain number might fall down and get a debuff, which means you will need to rest.
So to me this system sounds rather good and it allows combinations pretty much to your liking. Personally I will Increase lore to 10 on my main for dialogues and push athletics and stealth to 3 or 4 or so.
With another character I will push Athletics to 10, with another one I will focus on both Mechanics and Stealth to use him as scout. And then I will also take one character and increase his survival skill.
Again your choice in dialoges is limited to your main character anyways. So regarding the "problem" in W2 where you always have someone for every skillcheck: this wont happen, as yo can only use the main character in dialogues and it's simply not possible to have all skills high enough.
Then again it's also not always the best solution to use these options in dialogues.
Actually they can mislead you into bad decisions. Lets say you got 2 options to be diplomatic and one [Might] option to intimidate. This [Might] Option might be tempting but it might be the only option which leads to a fight or has a "worse" outcome than the others. So you have to think for yourself and should autoselect anything whenever there is an [Attribute] infront of it.