No these systems tend to be custom due to cost, reliability and size constraints. More so for GEO satellites which tend to be the most expensive.

Is this even true?

https://gomspace.com/shop/subsystems/attitude-orbit-control-...

https://www.cubesatshop.com/product/ifm-nano-thruster/

https://www.vacco.com/space/chems-overview

That's what I was thinking too. Those are all for Cubesats tough, none of which have gone into GEO so far.[0]

[0]: https://space.stackexchange.com/a/22721

I didn't research every spacecraft but if you look at GTOSat, its intended orbit is GTO, so it won't really need the services of Epic since a rocket can place them there directly.

And if you look at the spacecraft, it doesn't seem to have any kind of meaningful propulsion system. https://space.skyrocket.de/doc_sdat/gtosat.htm

I'm sure the other spacecrafts have a similar design concept.

Operating in GTO is terrible for electronics.

And also, we could do GTO 2X cheaper than large rockets for even their nominal payload size. For smaller payloads, it's more like 10X.

So is your idea to have spacecraft go to LEO first instead of direct GTO? Is this an efficient use of fuel?

It's not a detour, you have to pass LEO to get to GTO. Instead of the rocket pushing it to GTO the tug will take over. So the rocket has more payload due to only having to go to LEO.