I complied a list of 'Christmas lessons learnt' last year as it was quite stressful for a number of reasons, and I swore never again and to be more prepared this year.
My key takings from last year were:
Which Christmas activities we missed out on because I left it too late to book a slot, which activities we did do and were worth doing again this year (Mostly not relevant as almost everything we wanted to do was cancelled!)
Book a Tesco home delivery slot for pre Christmas and in between Christmas/New year as I missed out on those last year
Do not leave any wrapped presents under the tree as the dog will rip them open, even if they are not food related!
Book a day off work in December just to wrap gifts. Most definitely do not leave wrapping of DS's presents until Christmas eve!
Tracklements Christmas Chutney is amazing, but hard to find in shops - buy a couple as soon as you see them! (Sadly couldn't get hold of any this year 
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Do not buy DS anywhere near as much stuff - presents from us and Santa coupled with gifts from big families on both mine and DH's side meant there were over 40 new things brought in the house!
I took all of this on board this year (obviously many things not relevant due to Covid) and it was the easiest most stress free Christmas I've had in years!
I still added plenty more learnings from this year too, so hopefully Christmas 2021 will be even easier! This years key points were:
Don't buy any wrapping paper again (Birthday or Christmas) just use brown parcel paper and paper tape, make homemade gift tags and wrap with twine. They look lovely and can be customised for the recipient, and far more eco friendly too.
I already have a standing order each month into a Christmas savings pot, so the cost of presents is basically taken care of, but need to increase this to allow for the 7 birthdays in Dec/early Jan where a few people get 2 gifts!
Buy all gifts on one credit card and pay off with the Christmas savings pot, this year I was purchasing on various debit/credit cards and constantly moving money around and paying different cards off!
Started buying gifts slowly in September, had almost all of them purchased and wrapped by early December which felt a huge weight off my shoulders!
Buy several of DH's favourite Christmas chocs as soon as you see them! Managed to get one box for his birthday in early November which he scoffed within a week thinking I'd get us more to enjoy over Christmas but never saw them again after that!
Ditto Christmas Jumpers/PJs for DS - just buy as soon as they hit the shops (even if that is September/October) as you can never get them closer to the time!
DH made ahead and froze gravy, red cabbage, carrot & swede mash for main dinner, which saved loads of time (and hob space!) and nothing tasted any different, so definitely do this again.
Made a list of any foods we really enjoyed and to look for again next year, and any that were horrible and not to buy again.
Finally, just go with one present each for Nieces/Nephews/friends kids from their wish lists. Usually I set a budget then try and buy as many things from their list for that amount, but I've realised this is not a good thing. DS ended up with loads of little bits this year, and the volume of new things is overwhelming for him, and also for me - so I don't want to inflict that on friends and relatives too, when we all have quite big families who are buying too.
Sorry that was the worlds longest post! I've added more ideas from this thread to my list too for 2021 