@jeffc We were a recent sponsor of some work on the QBO Migration Tool. As of ~18 months ago, it kind of worked but was pretty fragile. (And it is important to understand that this was only a one-way, one-time migration.). One way syncing is much harder and Two-way syncing is probably out of the question.
At the time, we had modest interest from a client but that never got to the point where all parties were motivated to get “into the gears” and make the tool robust enough for regular use.
Currently I am working with two clients who are both switching from QBO. But I doubt that either of them are going to be interested in “migrating” and will choose to just “cut-over” instead.
At some point we will probably revisit this, but not for at least 6 months. If we do take it up again, my inclination (and I am not very smart about these things!) is to start with an Account Mapping Tool so that QBO could push their histories into a standardized CoA. I am pretty sure that the most recent version needs to be run on a clean-install and it writes a QBO-style CoA.
Sorry we can’t be more help. It is a deceptively tricky problem.