Hi OP,
there are lots of comments here so forgive me if I am repeating anything.
It is hard as a parent to get a read on what to do. My children are older and at a private school and during covid we had parents knowingly sending their children in with Covid while other parents were following the rules religiously. Even with children older it was hard to figure out what to do and the same is happening in this thread.
I know parents who have been ridiculously over cautious and equally, I have had a child who has slept in my house on a school night, has thrown up everywhere and the parent asked me to deliver her to school anyway (hard no there especially when she mentioned she had been sick the day before!).
Everyone has their thresholds for illness and some of that comes with agendas such as other children, busy jobs or at the other end of the spectrum, being more cautious because of family history with illness or shielding. Proximity to school is also a big factor. I live two minutes away and work from home. If they have a cold and they are warm but their temp is below 38 degrees and they seem ok in themselves, I send them in but let school know I'll be there in five minutes maximum if anything changes. Other parents who live out of the city have their children home a lot more when they are less ill because they have an hour journey each way and it's just too much. Saying that, I have kept my children off with milder symptoms if it is the day BEFORE a school trip to give them some chance of recovery (and to spread less germs round everyone else) and have gone in more unwell than I would normally send them in because it is a school trip and they will be outdoors all day anyway.
My advice would be to make an appointment and speak to your nursery. Flag the 68% attendance and explain why you have been keeping him home historically and that you are not sure whether you have been over cautious. Ask them to give you a guideline of when they expect him to be kept home, what they class as a fever or too ill to be there.
Moving forward, if you are unsure, call them, outline the symptoms and ask them whether they would be happy to have him in. They have the experience to know what to do.
When it comes to your time, as others have said, if you feel that he has the energy and that taking him out for the day won't result in extending his illness, then go but mentally prepare yourself to come home early if he flags. Then the decision is whether it is worth the money for a full day ticket if you end up leaving early but the odds are that the excitement of the day would carry him through anyway.
From what you have said, I think that you just need to know what the parameters are or as I like the call it the "panic point". You have had an unfortunate start to parenting with 2020 but I think that you are more than capable of handling this and I don't think a bunch of strangers on the internet can provide what you need. Our circumstances are all too different.
My two pence worth is to talk to your nursery and get their opinion on what to do when he should be there. I think that the guidance on nursery days will help you with the rest. They will appreciate you acknowledging the low attendance and asking for advice on how to improve it as every teacher I know loves a parent who is willing to ask for advice and acts on it! Good luck!