When cannon upgrade technology is added to the game, like via Rampant Arsenal, the Cannon shell (75mm) gets upgraded, however the Cannon shell (88mm) and Cannon shell (128mm) do not, so they seem to be using a different tech upgrade path, however because of that they very quickly get outmatched by the 75mm and thus become essentially useless. Any chance they could use the same cannon upgrade category as the normal 75mm cannon shell?

"Cannon shell (75mm)" is actually the vanilla cannon shell. I just added the numbers to make them look fit with my additions.
I have not played Rampant Arsenal (abbreviated as RA below) to verify this, but it sounds like RA has altered/added some upgrade levels to the vanilla category. Right?
Modifying from my side would mean cross-modification, which is bug-prone. So I guess it is better to add on RA side. (In this case, this would mean "TP adds new categories" → "RA modifies/adds tech about vanilla cannon shell category" → "TP modifies RA-modified tech"... Does not sound a good way...)

So please ask RA author about category names of my (auto)cannon shells: "autocannon-shell", "cannon-H1-shell", "cannon-H2-shell". RA just have to apply what RA has already applied on vanilla cannon shell category, on these categories.

but it sounds like RA has altered/added some upgrade levels to the vanilla category. Right?

Correct, it splits the vanilla upgrade tree in to multiple trees.

So please ask RA author about category names of my (auto)cannon shells: "autocannon-shell", "cannon-H1-shell", "cannon-H2-shell". RA just have to apply what RA has already applied on vanilla cannon shell category, on these categories.

Will do, done at: https://mods.factorio.com/mod/RampantArsenal/discussion/60034a84fccdb0dbc478c0f1

Thanks much!