Let me answer my question myself. I found the answer at the Max Mega Menu web site (though I had to squint to read the pale grey font over there).
That page explains why there is a limit of about 90 items on a wordpress menu, and installing the max mega menu plugin won’t change that. The page goes on to kindly explain how to increase the limit by adding bits of code here and there. Thank you. I’ll try it after a good sleep.
Especially since I’ve realized that I have another web site that I had better do it to, which also has a big menu that may explode any minute.
Too bad someone doesn’t invent a plugin to fix this by auto-adding the necessary snippets.
Thank you for the advice, though. Much appreciated. The lack of this information actually caused me to change web hosts a few months ago when the menu on the same site blew up the first time. Too bad web hosts won’t just tell the truth about it, might save them a loss of business, as it looks like they have poor servers, or vandalism, and the truth would spare you from moving out, and them from losing a customer.