The better you define the problem, the easier to define the solution.
From what you say, everything seems to work well UNTIL you engage the starter.
So, start from there. Check the starter relay. Check that the starter electrical connections are clean, shiny and tight.
Make sure your grounds are all clean, shiny and tight, also. All of them.
There's not just an issue with volts, it's amps, also. If there's not enough amps flowing thru the starting routine you'll get a lesser voltage drop.
The amps are getting used up by all the bad connections.
Remember the issues with bullet fuses, when bad connections would cause dim lights AND hot fuse holders, yet not burn the fuses? Same kinda thing.
Mine started slow but OK, with a 9 volt reading at startup, when I first got it.
The main ground from the battery to the frame was a sorta loose rusty greasy blob.
Fixing that also fixed a lot of other problems I was getting used to having.