It's really a tough call. I can say I've never been closer to just switching back 100%, but I also enjoy having both systems. Although I like the Z9, I have a lot more fun with the a1, so that's part of it. I just really like that little camera.
As for the lens, I think it's on par with the 600 GM (with and without TCs). If I liked the Z9 as much as the a1, I'd seriously start thinking about selling my Sony gear. Having the TCs built-in makes a big difference - I can't tell you (would be embarrassed to say, really) how many shots I've missed while messing with TCs.
However, I still don't feel like the Z9 is there - it's certainly not there from a customization standpoint. For me, I always feel like the a1 is actively trying to help me get the shot while the Z9 feels like it takes more effort to get the same results. If that makes sense.
If I were you, I'd rent / borrow / try a Z9 again and see what you think of it before selling anything. IIRC, you had a Z9 and weren't overly thrilled with it at the time. Firmware was helped though. I'd just hate to say that you should ditch all your sony gear and switch without your having tried the Z9 with the newest firmware first.