Reverse thrust flare transition too abrupt, with rapid pitch up

First, let me give a little background on our aircraft: I have a pixhawk-based flight controller, running a (slightly) customized build of arduplane 3.5.1, that includes a bit of code for deep stall, but the deep stall code isn’t being used. It’s a high wing, pusher, rudder and stabilator controlled airplane. Unfortunately the airframe isn’t as stable as I’d like in deep stall, so I’m investigating alternative methods of steep landings.
I set the speed controller and autopilot up to enable reverse thrust for landing. I am not using a rangefinder, and I’m not using an airspeed sensor. Not ideal for reverse thrust, I realize, but that’s just the existing setup and I’d rather not change it. Anyways, I’m able to achieve somewhat stable descents at ~25 degree angles by balancing my THR_MIN parameter with the descent angle. It tends to exhibit some stair-stepping behavior, overshooting the descent and then leveling out to get back on glideslope, but I’ve gotten it to a point that I think is acceptable.The airplane is accelerating to 13 m/s from a cruise speed of around 10 m/s, but I see airspeed variations that big during normal landings, so that’s not such an issue.
The main problem is that when the airplane enters the flare, it’s abruptly pitching from ~-25 degrees to +15 degrees. In looking at my logs, it appears the nav controller is commanding a pitch up to at least 10 degrees. This is causing a pretty big balloon in the flare, and by the time the nav controller recovers and brings the nose down the airplane is already stalling and rolling over. I’ve tried increasing TECS_LAND_DAMP, decreasing the LAND_PITCH_CD to a negative value, and increasing the TECS_LAND_SINK, but none of these are having much effect on the transition from steep approach to flare. Does anyone have any smart ideas on how I might be able to get the AP to quickly bring the nose up to level, without overshooting to a very nose-high attitude?
Here is a link to the T-log from a ~45 minute test flight where I’m adjusting some parameters and flying repeated reverse thrust approaches. The last three all activated the flare and exhibited the same rapid pitch-up behavior. The very last approach resulted in a stall and a cartwheel landing that broke one of my wings.
Thanks for any input you can give me!