I'm a bit on the fence with this one.
Absolutely reasonable to bow out on account of changing circumstances/priorities and not being able to afford it. They don't sound like very nice people and it doesn't sound like it'd be a very good time.
However, I also think if you're someone who is routinely late (and it sounds like you are and they don't trust that you will get there on time) they're probably a bit sick of you being late and/or worried that you won't be there on time. As much as you say that it won't spoil anything if you were late-that's not true, is it?
If they've planned a bottomless brunch for x time, (just an example) many of those come with a time slot and a limited time allocation so you being late would impact that. I think it's annoying when people act like their lateness has zero impact on anyone else.
OTOH, I absolutely understand your reticence to attend, especially if they just keep adding and adding extras on and then pressurising you to attend them all-despite your concerns. I hate it when things get agreed at the start-and then other people add things in that they can afford and don't consider if everyone else in the group can afford them, too.
I understand plans can and do change-but I think if those changes are going to increase the costs on everyone by a lot then that is a discussion that requires agreement. Or else understanding that some people may need to bow out of the whole thing or only attend the parts they can afford.
Just adding stuff and then demanding attendance is ridiculous. I think you should have been more direct from the start-probably making it clear that you have X budget and that is it-and anything over that you can't do and don't allow them to railroad you into things.
At this point, I think your only option is to be honest and back out. It'll be awkward and tense if you don't especially after those nasty messages. Be polite, and just cite a change of circumstances but you wish them all a good time.
It may end your friendship-but do you really want friends like that?