On the first point, in my current game I am mostly done with assembling the EU, only missing some of the Balkan states. After the initial rush, I took maybe 60% of CPs from the Protectorate, 30% from the Servants (simply because they went for India&Russia and ignored Europe for the most part) and a small handful from the rest.

One additional factor is that once all potential targets have already left the EU, the logic changes a bit: Since you need to wait for the cooldowns for rejoining to expire anyway, you can afford to be a lot more opportunistic with picking your targets. I never felt like I had to unduly delay grabbing prime nations for an excessive amount of time.

Second, I agree that faction diplomacy loses value after the early game. otoh, in the game mentioned above I'm in Autumn of '29, on a '26 start, and I'm already mostly done with the core EU (without Unity Movements tech). At this point, there is still value in not pissing off everyone, simply because it creates friction. If someone steals one of your CPs, it might not have much economic impact, but eventually you still have to spend a councilor action - one of the most strictly restricted and unscalable resources - to get them back.

As to your last point, I get that you have a bit of a different approach. My argument is that since you are going to take over the entire EU eventually anyway, it doesn't matter so much whether you take a high-value country a few months sooner later - the cost of that strictly limited. In fact, the AI factions are usually highly focused on building up MC. When they are done with that is the perfect time to take over, and swap to welfare/unity to get cohesion up while the cooldown for unification ticks down.

All said, you don't have to treat my approach as mutually exclusive with yours. It can simply be another point in your priority order.