Welcome. You ask a good question that brings up lots of deeper issues. It’s going to be very personal for each of us, right?
To get to the heart of it, do you enjoy the actual experience of riding indoors using Zwift? When you get off the bike at the end, do you feel better, happier? Maybe I’m weird*, but I find that I enjoy indoor cycling at least as much as doing it outdoors and am motivated by knowing that I get a mental and physical boost out of Zwift each time.
If doing indoor riding feels like a chore, the motivation will be much harder to find and has to come from some external factor. (BTW, if that’s the case, are there practical things you can do to make the experience more enjoyable? More comfortable, simpler and more immersive?)
Like you, I have all but exhausted the badge hunting. The equipment acquisition aspect that @Siobhan_Sinclair has highlighted has been good additional motivation for me, as is the act of progressing up the levels for the sake of it. Having those carrots in front of me does help push me to put in further time and efforts, even if the resulting value exists only within Zwift.
At this rate, at some point in the next six months or thereabouts, I’ll run out of new levels and I suppose I’ll just rely on the enjoyment in riding and the social aspects of Zwift. Still, there are many people who have reached the highest levels (25, then 50) years ago and who still ride a huge amount in Zwift as well as outside, so I guess they are all finding the motivation somewhere. I think that many of them are also racers, like @Gordon_Rhino-Racing points out.
The eternal optimist in me says that Zwift might surprise us all and soon add more challenges, more missions, more levels and other aspects that will provide lots of additional, external motivation… but we cannot rely on that.
(*To further illustrate my point, back in simpler times , I remember happily riding indoors through a long, snowed-out winter using the dumbest of dumb trainers for hours at a time with nothing but a blank wall in front of me and a Deep Purple CD on repeat. Yeah, I’m probably weird. )