The Crop_rec_4k.2018jul22 for Fir. 1.1.3 is stable and (I believe) still running on most cameras that are using lossless builds.
The same build for fir 1.2.3 can cause some memory errors caused by RAW_twk and MLV_play. Which does not brick the camera, but it makes you stuck with that build until A1ex fixes it. I am stuck on this build but I can still use both my cameras except for playing MLV in camera, which will turn up nasty error messages and A1ex's recommendation for me was to stay away from anything that initiates these errors.
Not being able to play MLV in camera is not a big deal for me, as you can't judge a performance by it anyways, you'll need an external recorder for that. Though, it is unfortunate that I can not test Danne'e 4k Builds, they seem very interesting.
Which brings me to Danne's 4k builds.
https://www.magiclantern.fm/forum/index.php?topic=23041.0 They offer even further tuning of the 5D3, like Pseudo 5k Resolution and other very interesting stuff, but I have no first hand experience with it and I have no idea if it is stable or a deep menu dive for every operation.
Danne's Batch_MLV works 100% with Lossless 14 bit. I do recommend you get the newest version of it though, as Danne added the -skip--xref option which will make sure that all MLV audio is converted, this was an issue before where none or only parts of the audio were in the final output.
Newest Batch_MLV Build:
https://www.magiclantern.fm/forum/index.php?topic=10526.msg215946#msg215946Good Luck and as always, do as much reading as possible beforehand, read the thread on Danne's 4k build for the ins and outs and ask around if you are in question of something, this is a forum after all.