I hear you OP - my husband is in the Forces so away a lot. He's as involved as he can be, but not physically here, so a lot of it is down to me.
I also remember being shocked at how much MORE there was to deal with when my kids started school.
Things I do, already suggested are having both a calendar app & a paper family wall calendar. The one with sections for each person is a lifesaver.
I also always have a pot of £1 coins by the door now, after being caught out too many times by non uniform / charity days.
Every time an important message from school comes in, I screenshot it and put it into an album marked "important" in my phone. Passwords for Google classroom, sports days, meet the teacher etc etc (tip from my son's teacher!)
Meal planning also really helps, but I have a "master list" of foods we all like. And (fully aware of how over the top this sounds) I have themed nights for food.
On the master list, I have a list of meals from that theme. So Mondays is always Italian. Pesto pasta, spaghetti bolognese, risotto etc etc.
Makes meal planning so much easier as I can never remember what foods we all like when I sit down to do the food ordering?! 🤦♀️
Making any packed lunches / snacks etc the night before then putting them in the fridge overnight makes the mornings run a bit easier too. I do this through the holidays too, makes life so much easier (and keeps me in good habit!! 😄😄)
The kids each have a basket that I (and sometimes they, but it's hit and miss!) put their uniform into for the next day at bedtime. Each morning they put their PJs in it, and just keeps things organised and tidy.
Lots of getting the kids to help as well - emptying lunch boxes, putting homework on the fridge...which I stupidly thought would help...but usually makes things trickier. But they love to "help"...!!!
Mixed into this is a lot of having to decide to miss some schools events, that I just can't get to, realising the house is too messy and guilt as well!
Sending solidarity as it's crazy how much juggling and keeping up there is once your kids start school. Once they've been there a year though, you do get into the rhythms of what happens each year which helps.