Usually, in the past, when I switched camera brands, I needed to replace all of my accessories, including my flash units...

However, when I added the mirrorless Sony A6500 to my kit which included the Godox AD 360ii flash and other Canon dedicated Godox flash units, all I needed to do was to purchase a Xpro Transmitter for Sony and I can use the Canon flashes (albeit in manual mode) with the Sony A6500...

To tell you the truth, I have not tried the flash with the Sony using TTL since, I tend to shoot off camera flash manually...

To do fill flash for portraits; I get the flash exposure down for the background using the f/stop and shutter speed that I want (which outdoors is quite often the widest aperture on my lens). Once I have determined the exposure, I don't adjust the camera exposure anymore. Instead I then turn on my AD360ii and Xpro transmitter (with flash in manual and with High Speed Sync Selected). I fire a test shot with the flash in manual... The power I select depends upon the distance between my flash and subject as well as on the modifier that I am using for the flash.

Usually, my first shot is between 1/16 and 1/4 power (depending on the parameters mentioned above)... I then adjust the power so my subject looks good... Here is a quick youtube video that does a better job explaining the technique than I can...
https://www.youtube.com/watch?v=50-1kK9bot4&t=6s

In the past, I tried to measure the flash using either a meter (doesn't work very well with HSS) or using guide numbers in an attempt to get the combination right on with the first flash. Separating the exposure for the BG and the flash fill for the subject into two distinct groupings is for me, quite a bit easier...