My understanding is that Siser heat transfer vinyl is urethane based (no chlorine) and as such is safe to cut in your machine. It’s available at your local Michaels. Whether the adhesive will stick to the glass without heat setting is something I don’t know.
The newer cricuts are faster. I have a first generation explore (no “air” so no wireless capability), the newer models have upgraded hardware and go roughly twice as quickly.
It has its limits in terms of detail it can manage, kind of like the GF with its overburn issues. With the explore, it’s a physics of a drag blade thing. I’ve said it before, if you’re good at a Cricut before you get a glowforge, you are waaay far ahead of the game.
Likewise, you’ll feel right at home with the cricut. They are very similar in terms of workflow and limits that they really complement each other.
I have found that slower is better with all kinds of cutting. If I am not in a hurry, I cut vinyl at about the third of the maximum speed my machine can do. It buys you a little safety margin and seems to be a bit more tolerant of a blade that isn’t at its best. If I was only cutting the occasional item, I wouldn’t spend much extra for more speed.
KNK makes good machines. I have the (discontinued) KNK Maxx Air and I have been very happy with it. It’s churned out some pro-quality jobs for me and has a place next to my GF.
Silhouette also has a pretty good share. I’d pick most Silhouette products over their Cricut counterparts (having owned both), except the Maker. Haven’t used it, but I have a Curio and I’m not impressed. I suspect the Maker is much better.