This is all based on my experience as a sushi chef, this is by no means definitive and please correct me if I'm wrong (all you pros out there)
The ability of a deba in breaking down fish, both round and flat, is more dependent on the cook's skill than the knife itself. In the right hands a deba can do amazing things. I find it is more versatile and easier to control than a western fillet knife. However, both types can get super clean fillets. The deba has quite a learning curve, like all JSBK, to become proficient. That is the part that really matters.
My suggestion is to watch (and study!) those videos gconcept999 posted and the videos from Japanese knife society, get a lower-end or used deba, a few different fish, and have a go. The ones we usually train new guys on are mackerel and striped bass, as they are a good size, economical and easier (IMO) to work with. See what it feels like, how it works for you and practice practice practice. BTW, in my experience San-mai oroshi is simple compared to Go-mai oroshi. Try flat fish later on when more proficient.
Steering shouldn't be an issue with a deba used only for breaking down fish, as the very shape and design of the knife are meant to facilitate the techniques it requires. The types of cuts executed on fish are generally to either separate the various parts (i.e. head and guts from the body), to remove the fillets from skeleton, remove bones from fillets (like the rib bones) or to break down hard bits like the skeleton/head into useful sizes. Generally, the knife "runs along" some line or surface, with one side staying put while the other is separated and/or removed. There are nuances in how to cut through bone, like what part of the edge one uses and what points in the skeleton one targets. Also, one's wrist needs to develop strength and stamina for a heavy knife. Deba are used at various angles and directions as a butchery tool, thus demanding flexible wrists, elbows, and shoulders.
I have only had to overcome steering issues when using single bevel or asymmetrically ground knives in general food prep (chopping, slicing, mincing, etc.) such as that of usuba or gyuto. It was usually when a single bevel or highly asymmetrical knife travels through a big stack of product on a board. To learn to deal with this, for a while I used a Shun 210 yanagi as a line knife for everything, veg meat seafood etc etc, and this forced me to learn to correct for the steer. Instead of looking at the bevel side of a JSBK, look at the hollow ground back side as the line guide for a cut's plane. (if that makes any sense).
As a lefty, using a mirror image of the righty process/technique should work. And there is a lefty yoshikane on JNS, that at 185mm and less than $200 bucks shipped, might be a good place to start. Best wishes and good luck!