Not sure if I have got your point here - But I am interested in what you have raised.
Just a question.....If you intend to embrace theses previously "missed things" like Xmas/ Birthdays/ Halloween/ Valentines etc etc are you not simply taking up in the boxes of the originators and inventors of these above things all over again? They too are / were in boxes of belief systems - So like the little Russian Matryoshka Dolls you are still inside one of them who is still in another etc etc etc.?
What matters is that these are opportunities to enjoy life and bring people together in a positive way. Most of the big celebrations, no matter where you live on the planet have a basis in the passing of the seasons. The rebirth of the sun, the harvest etc, have celebrations attached to them. Naturally, when people believed in supernatural forces they would associate these natural events with the gods. So if you want to think of the origins of everything you will find this.
I went to a great Diwali celebration once in India where they threw around fireworks in a rather alarming way, but everyone enjoyed themselves. This is called The Festival of Light and is it associated with the darkest time of year where a light festival acts to challenge the darkness. There are gods associated with it. It is a fun time and enjoyed by all besides those who want a good night's sleep.
Or what about the Hindu Holi festival which is again, a seasonally based celebration of rebirth and spring. They throw around brightly coloured paint at each other which I imagine could be fun. Again Gods are attached but, these are the same seasonal celebrations that exist everywhere. In Thailand they have Songkran - which i was lucky enough to go to - where they have massive water fights everywhere and everyone is carrying water pistols and buckets. It is basically the same thing as Holi and happens at a similar time of year to mark the same event. rebirth. They have different religious beliefs associated with it.
My point is, why does it matter? These events and celebrations - in the west, it is Christmas and Easter - simply serve the same function as other festivals elsewhere. They bring people together, make them happy. They serve a certain ritual service in marking the passing of seasons. Yes, if you go back far enough there were primitive superstitious people who associated gods with the natural events. If you go back a bit further there would probably be different gods still. But why should that bother us now? Just accept it as an opportunity for joy - there's no need to overthink it. Partake in the rituals as they stand - its not important where they came from, any more than knowing the origin of the wedding ring. If you want to throw off all pagan influence then you'd have to get rid of the calendar too as all the months of the year have pagan associations.
The OP is correct. It is great you have the opportunity to enjoy these times you missed with grandchildren and I am sure you will find it healing.