Peter, there may be more of a backstory than you are thinking. It starts with the first sentence in your post:
"Questions for those who have had systems professionally installed.
As you know, I installed my LiFePO4 house battery bank myself. I had contacted several people who advertised or stated that they were qualified to install, so in that respect, they were claiming to be "Professional". Two of these "Professional" LiFePO4 installers actually came out to the boat to work up quotes for my LiFePO4 install (and upgrade of associated equipment to support the LiFePO4 battery bank, like correct inverter charger, tie in to solar panels, etc)
When discussing the install with one "Professional" installer, . . . . he must have been a "Professional Marine Electrician" trained in LiFePO4 installs . . . it said so right on his business card . . . . anyway, I was grilling him on the batteries he recommended. I asked him: "Does the BMS have the capability to connect to my Cerbo GX, or through Blue Tooth, or is it standalone?"
His question back to me was: "What is a BMS?" I moved on to the next "Professional Installer"
Similar story with him. The questions I got from these "Professionals", and the answers I received from them when I asked for their "Certifications" varied from the useless, to the ridiculous. One guy (3 years ago) stated that he had been professionally (there's that word again) designing and installing LiFePO4 for the last 30 years!
Bottom line, Even ABYC at the time (3 years ago) didn't even have a certification for LiFePO4 training. I know, I called and asked them. They DID give me the names/contact info for three people on the West Coast who they stated were "knowledgeable" in LiFePO4. The one (1) in the PNW was booked up solid for two years.
I ended up leaning on my training as an aircraft mechanic (A&P), which I worked as, as well as taught, and did a LOT of research, and came up with an install (three years ago), which happens to be in compliance with the latest ABYC recommendations, but . . . per insurance companies, I know nothing, and the install was NOT "Professional", and therefore not insurable.
I realize that the new ABYC recommendations for LiFePO4 are now out, but to date, I don't know if ABYC, or any other true certifying body had come out with a "Professional Certification" for LiFePO4 installations.
So up comes the question again. Just what the heck does "Professionally Installed" really mean?