Moving from street photography to automotive videography

First : I think you undersell the performance of APS-C cameras when it comes to video. There are some excellent APS-C options for video and your X-T3 is one of them. It has access to 10bit video which you can use with F-Log (which is a decent Log profile). It has a variety of codecs and record modes that will allow you to explore a lot of different things when it comes to videography.

Second : get a gimbal, this is unvaluable for car videography, especially rollers.

So, in your shoes that's what I would do : get a used gimbal (it's pretty cheap nowadays), start doing video work with your X-T3 (which is still a great videocam), that will give you a baseline of the things you actually want in a camera, and that will help you better for your future camera purchase than asking a forum to choose for you. And who knows, the X-T3 is an awesome video camera with a lot of video features, maybe it would end up being enough. And if not, that gives you a better understanding of the things to improve.

From the get go, I would say the S5II is the absolute highest value for a videographer, but it's neck and neck with the Sony FX30 in my eyes. The key point of these cameras is the ease of use of the interface for video, and the active cooling that allows you to record in the most intensive video types without overheating.

Thank you for the response. X-T3 actually did impress me with how good the videos turned out. The things that made me consider to change the system were: after I couldn’t make the autofocus stay constantly on a moving car, but I will have to tweak more the AF settings maybe; and that after buying a gimbal (if i can’t find a good deal second hand) for the entire cost of my Fuji gear, i could at that point just switch to a system better designed for videos.

I’ll keep an eye on some gimbal deals near me, worth testing before making a switch.