First, we need to ask what country you are in, as the device selection does vary somewhat.
That said, you are correct that there are no G8 Smart bulbs that I am aware of. There just really isn’t room for a radio in that format.
There are multiple smart switches that you can use, however, and some of them fit over the existing switch and then mechanically move the switch underneath. So those can be used with any existing dumb bulbs because you aren’t going to change the wiring and you aren’t going to change the existing switch.
They do stick out from the wall quite a bit, though, so you have to think about whether you’re OK with that.
Again, though, we need to know what country you’re in.
If you are in the US, the Third Reality Zigbee Smart Switch Cover has an official SmartThings Integration and works quite well. I have a couple of days at my own house. (US) the only thing is that they are fairly noisy because they are moving the switch underneath and you can hear the motor when you turn the switch on or off. Not terrible, about the same as a typical smart lock, but not like some of the other smart switches that don’t have to do this mechanical movement as well.
It is sold on Amazon with its own hub or individually. You don’t need their hub if you have a smartthings hub, smartThings can act as the controller instead.
This lists for $39.99, but seems to go on sale fairly often.
Ecolink has a similar battery powered Z wave device (US only) which physically moves the switch underneath.