AHA. The key is the formloom.js file, whatever it’s named after it’s published on your server needs to have “var timeout” and make it = 120000. That is the key to getting the SUCCESS message.
The only problem is, I upload this website every other day, so I have to change this manually after every update. Which is a pain in the (in Forrest Gump’s voice) buttocks, sir. But we are getting there!
Indeed! Guess what? One of my sites has seven forms on it. That’s seven formloom.js file edits, post-publishing. Anyway, @yabdab has said he’ll be adding this setting in a future update. Presumably when he releases the M1 native version 
btw @Steve, I don’t believe PHP7.4 is deprecated, or indeed 7.3. But 7.2, yes.
php 7.3 and 7.4 are not end of life just yet
You’re right, in my nervousness I simply read it wrong. Thanks. I upgraded Sendy from 5.2 to 5.2.1and it works from 7.2 to 7.4.
But I only got Formloom 4 to work through a kludge on 7.2, 7.3 - 7.4 don’t work.
Probably just PHP settings and not the version being used.