I can say two things on this (well my opinion that is)
Fruitjuice is useless af far as battery health goes, no need to calibrate etc…
Start fresh: i had problems updating my 2016 MBP to catalina and started fresh. It took a couple of hours to get back to almost full operativity (YMMV, depend on the setup), and got rid of a lot of cruft and never used app in the process; I’d not do it every year but with a new Mac after a while it’s worth it
Just went to the new 16" from a 2011-or-so 15" that had been through many upgrade iterations, in fact dating back to a 2004 iBook. I decided it was time for a fresh install. There are enough significant changes (no 32-bit apps) in Catalina that it seemed like the best approach.
And regarding Dongletown, I bought one of these: https://www.amazon.com/gp/product/B07FX2LW35/
For $38 it gives me back the old USB ports, SD card reader, and Ethernet ports that were taken away and gives me a HDMI. Solves all my connectivity in one neat little box.
My advice for living in Dongletown is to replace cables as you need them. I do have one VAVA adapter from Amazon which has 3 USB A ports, Ethernet, and an SD card reader. I use it as power pass through and Ethernet on my Air when podcasting.
I have been moving into a new MBP 16 myself over the past several weeks. I decided to take it slow and not hurry things along. If I run into something that is missing – e.g., a folder of custom DEVONthink scripts that I wanted to use in the fresh install of DEVONthink on my new machine – I plug in an external SSD that contains a clone of the old machine, find the data, and copy the data over.
In the process of moving I’ve abandoned lots of applications I hardly used, and a lot of cruft. The old and new machines are both 1TB SSDs, and through the moving process I have ended up with a significant amount of more free space on the new machine than on the old. My old machine was the end point of a long chain of OS X/macOS upgrades and migration assistant migrations from previous machines. By breaking the chain and starting over by building the new machine from scratch, I have a sprightly and slimmed down user account to work from, which is great.
I also used the Catalina 32-bit app abandonment as a reason to start over with a fresh machine.
Over a period of a few weeks, I only moved over applications that I actually use. No real problems, though some programs are wonky with their “deactivate your old install before installing a new one” routines (they’re never quite the same.)