You have had good advice on this thread- do the essentials that will genuinely make a difference to your child's progress, such as reading a few times a week and forget the rest.
I have never baked anything for a cake sale, I send them in with a few quid each and am a good customer!
Costumes are a pain, but our school is realising that now and there's much more non-school uniform days for money than dressing up days now.
It is also partly what you make it, I can't believe someone said secondary is as bad as primary, it isn't really- I haven't been into my child's secondary school since she started except for one concert.
Most schools now have Parentmail or Parentpay and if yours doesn't, worth asking if they are going to transfer to an email system/online payment.
As for the rest, just don't go. My dd can't go to sing in the carol service this year as it's an hour drive away on a dark night and I can't arrange it. She won't combust!
Make the time you have together count- so homework with a lot of support, reading, listening about their days at school, extra help if they need it in a weaker area.
It does annoy me too though, our primary sets far too much homework and doesn't leave time for me to do more useful stuff, it's also at the wrong level half the time.