Yesterday I also came across this limitation (8 minutoes in 4k video) with Nexus 6P camera but tried the Opencamera app (free) and it got past the 12minutes without problems (and it has more manual control for exposure, focus, bitrate) and after some testing with phone on a tripod I think I'm going to record my kid's X-mas party with bitrate of 20mbps because it uses half the space (I think default is 42mbps) and on a tripod it looks as good as default bitrate.
I've done another test with 20mbps and it stopped at 27mins and 4 seconds, I think file was already 4GB at this point, so maybe I could still get a longer recording time with 15mbps if needed (it is still pretty good quality for a tripod mounted camera)..
I'm not as happy with audio, I thought it was stereo but it's mono and I have the feeling that it is captured with MIC's facing wrong side of the phone... and at this point I have no time left for ordering external MIC's to test (but it should be mono as well, maybe better direction would give better quality in high frequencies - but that has to be next time). For now best option I think is selecting audio source to camcorder (I think it's the one that comes selected as default, but not the option called "default" and I did try to force stereo but file still gets mono audio but I hear high frequencies coming from rear MIC - not as bad as I thought) - I'd love to see an option to capture the audio from the 3 MIC's givving 3 channels audio (good to play on 5.1 systems using center, left and right speakers