Since the Librem 5 USA will be delivered in January-February with the PLS8, I think that we can safely assume that the PLS8 for the Librem 5 will be available in the same time frame. Purism is having to custom manufacture the M.2 card with the PLS8 in the U.S. (probably in the San Diego area).
See the FAQ: https://source.puri.sm/Librem5/community-wiki/-/wikis/Frequently-Asked-Questions#22-how-well-do-cellular-phone-calls-work-in-the-librem-5-is-volte-supported
While making a cellular call, the connection will drop from LTE to 3G. If 3G isn’t available, then it will drop to 2G. For this reason, some people have reported that their internet connection appears to drop while making a cellular call.
The PLS8 is more transparent than the BM818, but you are still using tech from Qualcomm. The PLS8 is based on the Qualcomm MDM9215 modem (28nm, 100MB/s max) from 2012. The BroadMobi BM818 is based on the Qualcomm MDM9607 modem from 2016 (which has 256 MiB DRAM, 256 MiB NAND, and a single Cortex-A7 CPU clocked up to 1.3GHz, and is the same modem used by the Quectel EG25-G in the PinePhone). Both the MDM9215 and MDM9607 are Gobi 4000 modems, so they are similar, but the BM818 supports a lot more LTE bands.
Thales has a public community forum where you can ask questions, it provides full documentation about the PLS8 to anyone who registers on their web site, and it focuses on selling specialized security products. In contrast, BroadMobi doesn’t provide any public documentation on the BM818, and I assume that Purism had to sign an NDA to get it.