I'd look into a ZChiller. I've had one for a few years, the large one. It's amazing how well it works! It's also all copper, with convoluted internal tubing.
https://www.zchillers.com/
Looks like possibly mineral deposits.
I'd seriously consider a scrub/polish with barkeeper's friend, then rinse very well. Follow up by a thorough washing out with PBW and a sponge to agitate off the BKF residue. Rinse very well, then let air dry for a day or two.
Also: This is why PBW beats...
If you're planning on any kind of pressure, finding someone to TIG weld it for you is probably a better idea. Most time silver solder is used with a tight-fitting flared opening and that's going to be difficult/impossible with a 4".
With the amount of solids generally present in a Hefe, I'd almost guarantee that anything carbed at high volumes, even if the bottles hold, that they'd gush due to nucleation points.
My Baratza grinder finally showed up.
My god, what an eye opener. What a difference a grinder makes. No fines, very consistent grind. Same coffee, same grind size, same weight of coffee, same water. Only difference is the grinder. Cup is 50% better.
You will still have suck-back, as the negative pressure is caused by the colder liquid absorbing more gasses, not just CO2. You also have the volume of the liquid itself shrinking due to the temperature change.
Because I like clear beer in the keg so I can move it around without stirring it back up? It's also more storage friendly without extra yeast/trub in there? It makes cleaning the kegs a lot easier when there isn't a bunch of mud in the bottom?
Need some more reasons?
You can both spund *and* cold crash without oxygen exposure if you have the correct fermentation vessel... that being a pressure rated vessel of some sort, not a plastic/glass carboy. You can cold crash while minimizing oxygen exposure with a reservoir bag of CO2.
Wow, that's a bit of an eye opener. Would think with the sulfates in the 125 range it would be very dry.
Might be worth talking about how the coffee is being made. I've been brewing inverted aeropress with a relatively fine grind, so full immersion.