Thanks, everybody for your help and insights.
@BluishHumility I had been thinking about Refind, but held back TBH. I don’t understand enough about efi and boot (though I have learned a few things on this journey) and I didn’t want to completely brick the system. Plus I had wanted to use Grub to be able to access snapshots within the Grub menu. I had a look on the Refind web page and decided that it was worth a go, as it looked simple enough to remove it if things went wrong.
Using rEFInd
First off, this appears to be a great workaround, not perfect, but frankly, close enough for me.
The author said that he maintained the PPA on the Ubuntu repo. Since installing .deb files (an uninstalling) is really simple using the package manager (graphical or command line) and the PPA is going to be kept up to date since it’s maintained by the author of the package, I choose to install it this way, after all I have both Arch based and Ubuntu based distributions on my system, I can choose. Installing was quick and simple as expected. Then I rebooted.
I was brought into the rEFInd graphical menu and it had about 5 choices for me, I still need to investigate the others out of interest (probably fallback mode or something like that), but the first two appeared to be Mint and Endeavour. I clicked on the Mint button and to my surprise, it booted in the Grub Menu… with my snapshots still in the menu! I checked it worked ok, yep all good and rebooted. I chose Endeavour and it booted into Endeavour’s Grub menu… when I can also set up snapshots 
Yes @BluishHumility I know you mentioned it would boot to grub, but I didn’t realise that before. I thought it would just be a replacement for Grub.
So, the bottom line is that I have 1 extra click on boot to choose my OS, but I can also have the btrfs snapshots in Grub that I wanted. I think that can be considered a decent solution.
Thank You Everybody.