Just a quick update.
I ended up using OpenHAB (1.9) as a shim between HomeKit and Homebridge. As somebody once said, the surprising part wasn't that it works badly but that it works at all. There were some minor issues along the way, e.g. dimming broke with iOS 10 so all my dimmers had to redefined as switches, OpenHAB push notifications got moved to a new server so it took some fussing around to get working on the new service, a change in macOS broke the startup shell script, everybody on the OpenHAB forums seems to be into advance stuff like MQTT so I feel like an idiot trying to follow things, the HAI/Omniscript binding has some limitations (but nothing that effects me), it lacks an active developer for either the OpenHAB binding or the Homebridge plug-in so it's always one change away from being dead in the water, and it's not at all clear how well it works with OpenHAB 2.0 which is now out of beta. But it mostly works and, despite what you might read elsewhere, all it needs is some copy/paste configuration and some trial/error to get working. Virtual servers are a godsend for this type of work (I'm using macOS VMs running on a Mac Mini with Parallels).
SmartThings via Homebridge is also working although I mostly use it for a couple of Z-Wave door sensors. Like OpenHAB, it requires some care and feeding, the most recent being this week when everybody needed an update to reduce the amount of traffic hitting the ST servers. That update required an update to Homebridge and, like an idiot, I decided that I should update node and npm, so it didn't take me long to turn my Homebridge and OpenHAB virtual servers into dumpster fires. But I rolled them back, limited myself to the essential upgrades, and I was back in business within a day.
The reality is that I lack the interest and skills to keep this precarious mountain of software and hardware running indefinitely so my plan is to strip it down as opportunities present themselves. HAI/Omni, Homebridge, SmartThings, UPB, SmartThings, and OpenHAB are all on the list of things I'm expecting to replace in the next couple more years. HomeKit, Lutron (switches and shades), Hue, Alexa, SimpleHome (for AV), Nest Protects, and Nexia (thermostat only) are the technologies I'm investing in now.
I'm not saying that these are the right technologies for anybody else (most of the people here would think that keeping that mountain running was at least half the fun of HA and want to hack it) but these are the ones that fit most comfortably with my skills and interests. Also, I think these are the ones (except maybe SimpleHome which is not so simple) that my wife wouldn't have to immediately pay to rip out if I got hit by a bus.