I don't agree on this particular point
I get that some people don’t celebrate birthdays etc but she celebrated her 40th and I spent money on the event she booked and I spent time/effort. If she’d not celebrated hers then fair enough not to celebrate mine but it feels a bit like a snub to be honest. It’s possible to make effort without spending cash?
What you are saying here, is that you went to the party / celebration event she booked to celebrate her birthday, and, as etiquette suggests, you took a gift, as you had been invited to a celebration. Now, due to COVID, you haven't yet been able to book a party / event. So it is quite reasonable for her not to have brought you a gift / made a fuss, as neither of you have done for each others birthdays for 20 years.
I was due to go to a 40th in May. Obviously I would have taken a gift (I don't normally send cards or gifts to this person, but I would have, as she was having a party and she invited me). Obviously it didn't go ahead, and there was various chatter about having a "41st" next year, or "just telling everyone she was going to stay 39 until she could have a party" etc. So, when she does have her celebration, if I am invited, I will take her a gift. I didn't go round on her birthday as I normally wouldn't.
Re the tickets - I can't imagine having an ongoing arrangement where one person continually sponged off another, tbh. I like to think that, for a good friend, as a one off in particular circumstances, if I could afford to, I'd treat them to an event, but as an ongoing thing, it is just odd. So I don't think it would be unreasonable to not treat her to this big event, but I wouldn't link that to not getting you something on your birthday when you haven't (for reasons beyond your control) yet invited her to anything.