that would just be "Action"
if feel absolutely necessary that it has a prefix name it would be a Standard Action, which is fine for describing it but from the players standpoint it just looks ridiculous.
every possible prefix would just look weird from the players viewpoint.
Main Action, Basic Action, Normal Action... none of it works visually, only for description purposes.
so would be best if just stuck with Action.
D&D recognizes this too. they give description prefix names to the primary action only when talking about specific skills (Dash Action, Attack Action) but when they list them in terms of "this is what you can do in a single turn" its just Action, Bonus Action, Free Action, Reaction
edit: i do think you should change "Assist" to "Support" though.
assist just sounds like "ill drop what im doing to help you" while support sounds more like "heres a buff/heal, but i still got my own s*** to deal with too"
(edit2: pointless discussion) as for the whole Instant Action vs Bonus Action.
Bonus Action is better for real time or actor turn based combat (ATB/wait combat systems)
while Instant Action is better for party turn based combat (pick all party members actions at same time)
they both describe the exact same thing. but in party turn based Bonus Action "could" also just sound like attack*100% trait (2 separate standard actions) to some silly people. obviously its not.