How does the ETRS compare to other models such as the SQ-A?
To me, the 6x6 cm SQ-A is a better 6x4.5cm camera than any of the ETR-series cameras. For not much more in terms of money or size, the SQ series offers cameras that never need to turn, are smaller in practical usage (i.e., because you don't need the prism and speed grip for portrait orientation and only need waist level finder), and have frame lines for 645 etched into the focusing screens.
Indeed, many photographers have used Hasselblads, SQ-As, and other square-format cameras to shoot 645, cropping from square. I personally prefer square, but the SQ cameras, as with other square-format cameras, easily offer both formats whenever you want them or you can buy a 645 back if you really need it.
Yes, the ETR cameras offer 3 additional frames per roll, but this isn't worth it for me given the additional size and weight the prism and speed grip add compared to the SQ-A.
All this said, I would have no problem using or recommending any of the more modern Bronica cameras (i.e., ETR series, SQ series, GS-1, and RF645). I've owned Hasselblads, Rolleiflexes, Bronicas, Plaubel Makina 67s, etc., and I've never felt let down by any Bronica equipment and their results. I have had to get some equipment serviced occasionally, but these issues were usually caused by me (e.g., dropping the camera hiking on the isle of Skye). The only issue I've really ever had with these Bronicas was that the Bronica SQ-Ai battery door and compartment are
very fiddly, so I would ordinarily try to go for the SQ-A or SQ-B where possible, which take different batteries and avoid the battery door issues.