For my addons, I start out trying to keep a single version that works with both 5.4- and 5.5+. But the second I run into problems, I give up and fork it, so the people on 5.4- are stuck with whatever version and functionality existed as of the 5.5 release, while all new development going forward will be for 5.5+ only. In other words, if it requires any extra work to maintain 2 versions, I'm just abandoning the old version (I suppose if there was some security vulnerability or major bug discovered, I'd go back and fix that in the old version, but other than that I'm done with 5.4- unless it just so happens to work the same as the 5.5+ version).

All that being said, my addons are not as complicated as yours (except maybe Designer Content, for which I did the forking thing and will no longer be putting any work into the 5.4- compatible version).

Nearly all my addons required none or only a few minor changes, mostly just adding css classes for 5.5 (leaving the 5.4.x classes in there) and they then worked in both. I also had to do some minor tweaks of JavaScript.

For Magic Heading I added 5.4/5.5 conditions that switched default scraping selectors between Plain Yougurt and Greek Yogurt.

For Zone Based Shipping the C5.5/eCom-2.5 shipping edit interface was not a big enough panel, so I moved the whole interface to its own dashboard page and made the eCom shipping interface a trivial 'See other page'.

All now have a single version for both forwards 5.5+ and backwards 5.4.x compatibility.

But then none of my addons have difficult interactions with C5 itself such as using scrapbooks or creating/modifying page types and pages.