Solar reg question

[the evil twin]

plan is to have 5mtrs of figure 8 with a merit plug to connect to the panel. at the other end
I'll have a 50 amp anderson plug off the batterry to the MPPT reg

I wouldn't use a Merit Plug. Something like an Anderson Powerpole would be heaps better.

Is there a reason you don't want to "hardwire" the Reg to the Battery?
You could simply have 5 metres of cable from the Panel with an Anderson on the cable end that plugs into an Anderson fly lead on the "Panel Connection" of the reg?

Simpler and less connections means less issues and less voltage drops