<< The upshot of this is the MBP will be on charge almost continuously, is this going to have any impact on battery health/life? >>
Your statement as written is not quite right. When connected in this way, power is AVAILABLE at all times.
The CHARGER is inside the MacBook Pro, and is completely under program control. Your Mac accepts some power when needs to run, and accepts more power when it intends to charge its internal battery. A power source like the Power Adapter or certain Displays can not 'force itself' on your MacBook Pro. Only the algorithms inside your Mac can decide when is a good time to charge the battery. Your Mac can NEVER be over-charged.
Any external power supply that provides "USB Power Delivery" (including certain Docks and Displays) must negotiate over the USB cable using USB Power Delivery Protocol, and can not 'force itself' on your Mac. The Voltage and Current are delivered only after your Mac requests and the power supplier agrees to supply power under certain controlled conditions. The computer draws only as much power as it wants, and the computer is in control of the entire process.
Battery Health Management feature tracks your usage history and detects that you do not generally run your battery down to low levels. So when you see less than 100 percent as a full charge, it is charging less than the highest level for longest battery lifetime.
NOTE that charging to 80 percent is a side-effect, NOT the GOAL.
The goal is to have some hurly-burly (percolating activity) around using your battery, either by your actually using it, or by leaving it less than fully charged, so that the cells are not 100 percent charged 100 percent of the time. Battery Health Management will also from time-to-time let the battery decline slightly to 'get some exercise' and run lower (thought to be around twice monthly) if you do not do so by your ordinary use.
Activate Battery Health Management and do not spend another moment of your time thinking about charging.
