I'm a very long time divorced but when we initially split, our kids were 7 and 10 so Christmas and Santa were still ongoing in our house at that time. Like you (I think?), I also felt a bit of guilt over him missing out so invited him for all the Santa years after that (up until youngest was about 13 or so). I also dreaded the days and I can honestly say I never enjoyed them but I did them for my kids. What worked for me were:
Giving him an arrive and a leave time (i.e. come at 2pm for dinner at 3pm, eat, watch Christmas movie, have dessert, then leave at 7pm)
Give him a list of jobs to keep him busy (i.e. put new toys together with the kids, play games, peel potatoes, wash up dishes - this was to prevent him getting the opportunity to just sit drinking and getting more maudlin - learned this after the first year!)
Don't let him 'bring' a dessert or anything else (also learned this in the first year) - gives too much free time and not enough jobs to do which can just raise the tension
No alcohol before or during dinner - maybe one after dessert if I could tolerate him staying
Separate rooms as much as possible to limit tensions (i.e. I made dinner while he amused kids, he washed dishes while I sat with kids) - meant the most part of being with him was actually being in the same room for eating)
A reminder any time he came into the kitchen to try and 'talk' (more a problem in the early years) -'I'm making the dinner, can you go back to the kids'
Stick to talking about 'safe' subjects (i.e. how is work, how are your family, did you see they are building a new motorway)
Any deviation from 'safe' subjects and there was a sharp reminder we weren't talking about that right now and if he persisted, he would have to leave
A half hour reminder before it was time for him to leave (i.e. 'oh, it's nearly time for you to head off - do you want another cup of tea before you go?)
I won't lie, the first year was truly s**t and was really hard and emotional but by sticking to my guns and not allowing any emotional talk, I kept it on track. The threat of kicking him out if he persisted was enough to stop him. As the years went by, it got a bit easier and he accepted the rules although I never enjoyed the day myself. I was always stressed (less each year but still) but it's a sacrifice I was willing to make - not because I'm some type of martyr (I'm not and I cracked out the whiskey as soon as he left!) but because for me at that time with my children the age they were, it was the right decision for us to include him.
Good luck to you.