Hi OP, I can see how anxious you are feeling about this, it’s a huge change and will be really difficult for you, but that feeling will settle some.
my exh and I started out with a. 4/3 week for a few years when our DS was young (3-7) then moved to a 5/2/2/5 as a pp suggested, for me that was the best version of 50/50, it meant as the ‘single’ parent I could do a class or have a standing weekly meetup with a friend as I always had Wednesday and Thursday nights to myself.
it also meant that I got some extra time as ds would come home to me for dinner on the handover days from my house and also one dads handover days from dads house, so I never really had more that a couple of days over the weekend without seeing him, plus on the weekend I always had the option of going to watch him play sport.
we have very recently switched to week on week off and it’s so much harder, my regular Friday morning walk and coffee with a friend before work is how fortnightly, I struggle to go to the gym in the mornings as I’m so aware of this being my only week with him.
I’m sure it will settle but for now it definitely is an adjustment.
regards to holidays, we are both flexible, I’m definitely the poorer parent 😂 so don’t really have holidays, but dad has remarried and also earns a lot more so does take ds on trips overseas, I try to see it as how fortunate ds is to have these opportunities, and as long as he is happy and enjoying himself then I’m happy too.
we do generally split the holidays into bigger chunks of time to accommodate this, even if my time is spent at home doing things together rather than away.
Xmas, things for many many years were extremely amicable and we generally would spend Xmas day together, even when he met his new wife. That has only recently stopped, and now we split it with a lunchtime ish handover. So one gets Xmas Eve and Xmas day morning (which let’s face it is the best bit) and I am happy for this, I don’t have any family at all so I struggle on Xmas day with being ‘up’ all day so am happy to then drop him at dads for the rest of the day and get him back a couple of days later.
the option for swaps, I always accommodated these for many years, when it became so often I had to say no more as it was so disruptive, there were more changes than not, so I suggest you don’t be too accommodating as the expectation can create stress and resentment. The default should be no unless the other parents really stuck or if it’s a work event which isn’t optional.