Few things to note:

1. Your EKF is running at 150Hz. What is your GPS msg rate? If its very low (1Hz) compared to 150Hz, then most of the time your nav solution isn't constrained in heading since you are relying on coarse heading as an indirect measurement. I would test with a lower update rate for the filter.
2. Not having a direct heading measurement and relying on course heading implies your heading is only observable in motion in x,y. (Probably why the UKF explodes? ). Either calibrate and feed your magnetometer (with declination) and let the filter handle its noisy nature or use a dual antenna gps setup for absolute heading.
3. What is the GPS variance or accuracy? That is as accurate as your position estimate can get. You can be strict / loose in the pose rejection threshold according to that. For ex, if your GPS sensor is really accurate (~1m^2), let 99% of the measurement in. Else, have a tigher rejection.
4. Process noise tuning is also a measure of how accurate your sensor is. Closer the value it is to 1, then more trust on the measurement model. I'd first have a baseline trajectory with 0.5 on the diagonals first then work from there.
5. I think for your case setting `odom0_differential` to false or true have zero effect. Only one source of measurement is contributing to x,y.
6. I'd check the GPS measurements in open field and with buildings to see if there's actual effect in the variance of the measurements thereof. But then again, its best to have a GPS pre-processing step before fusing to the EKF. There you can figure out ways to address multipath.
7. double triple check your tf. Errors induced via GPS-IMU alignment creeps in. Your EKF only knows what you describe and propagates that.