I don't disagree with your use of 8 AWG, that's a smart move, especially for future expansion.
I disagree with your assessment of Blue Sea. You are writing off an entire company of known quality based on your experience with one of the most inexpensive items in their catalog. It is built to spec, and better than it's peers. For a cig socket, id say that's pretty good considering the price.
As to your comments about twist lock, I get it, and know how most people would interpret it because I did the same. Apparently that's just how they are built across the board. There isn't standard here for plug/receptacle combo to make it a proper twist lock. Not really a Blue Sea problem.
I'm familiar with the charts... but they don't publish the voltage drop % either. It's not that the charts are wrong, it's just that they don't tell the fully story or what is acceptable for the application.
I have two charts that show 14 AWG (12.65% drop) and 8 AWG (3.14% drop) as acceptable selections for 20 feet of wire at 15 amps with both being on opposite ends of what's acceptable. I generally size between 5-10%.
The typical ARB 50 fridge uses about 60 watts running (about 5 amps) and has minimal startup surge currents. The problem was only a concern due to the very small OEM wiring and the contact resistance of these cig plugs. Doesn't take much to fix this. Sometimes just starting the engine to raise the system voltage is sufficient to eliminate the problem
I've spent 10+ years in DC power distribution and design. I've helped a lot of people on this forum with their electrical and dual battery setups as well.
I got no problem with overkill... but your experience hanging 8 AWG off a spade connector tab doesn't surprise me. I would have terminated that to fuse block, and used 14 AWG branch fused at 15A for the cig socket.
ST Blade Fuse Block - 6 Circuits with Negative Bus and Cover - Blue Sea Systems
At least now if your boat catches on fire... you can call a company in the US that actually has liability insurance.