Cicero’s educated Greek body slave, Tiro, invented it.

https://en.m.wikipedia.org/wiki/Marcus_Tullius_Tiro

https://creativepro.com/ampersand-history-usage/#:~:text=The....

I think you might be confusing ampersand with the Tironian et which looks like a 7 - Unicode point U+204A. It is still visible in Ireland on the old Post 7 Telegraph boxes. Tiro invented a shorthhand system and his Tironian et represented the sound "et". I recently dived into this whole subject so it's kinda fresh in my mind.

Maybe the second link i provided has it wrong. You seem to know more so I defer to you.

From the book “Shady Characters” by Keith Houston

"Among Tiro’s notae was an innocuous character representing the Latin word et, or “and.” Though this was only one symbol among many (in their most elaborate medieval form, a system descended from Tiro’s original cipher comprised some fourteen thousand glyphs), the utility of Tiro’s system ensured that his et sign would considerably outlive both its creator and its sponsor. This was not, however, the storied ampersand: when Tiro created his so-called Tironian et, the ampersand was still more than a century away."

The book itself is worth a read for all the other punctuation marks it discusses.

I really like this as an alt etymology. If it got popularized to save characters in telegrams, it could have been the “electric and”. Another way it could be from Ampere is that his son was a philologist and could have conceivably promoted the idea of an ampersand.