I know this may seem silly- but check for VERY MINOR leaks at hose connections. My X3 (58k miles, all stock at the time) was boiling the overflow when I would stop. After investigating carefully, and losing coolant slowly via vapor not fluid on ground, I found that the oring in the connection going to the overflow was not sealing fully, so when I would park with coolant at op temps, enough gas pressure would leak, that it would begin to boil sitting there. The fix is obviously to replace that line at some point- but tbh, since it's a vapor/gas leak not a fluid leak, I cleaned the connection sides, put some silicone grease on the oring, topped up coolant, hasn't done it since.
Another thing to consider: it isn't super common, but water pumps do fail on these cars. If you have ever run it low on coolant or like in this instance, boiled out the coolant at one time or another, forgot to bleed whatever and it ran dry it's quite likely to fail prematurely. So do some very careful hunting around. Any sort of leak that results in pressure loss during op temps will result in boiling, which starts the vicious cycle of not being able to properly circulate coolant/gas bubbles in the head/uneven cooling etc, all which make the original problem worse (and this is usually what leads to Hg failure in general in my experience as a mechanic, instances of cooling system failure. )
Also worth noting, (maybe not if you're tuned) that ecoPro or whatever targets a much higher coolant temp. 240 degrees I want to say? Where as "sport" lowers the target some. So if you've been using eco mode, I would discontinue that. If you have the ability to change your target temp in your tune, and won't be driving a lot of miles but NEED to get somewhere, lower your target coolant temps to reduce coolant pressure. There's a lot of issues with this though, one of which will be a faster than normal accumulation of condensation and fuel in the oil, so I would increase your OCI, but if you suspect headgasket- you should be pressure testing, and sampling oil for coolant contamination, and coolant for exhaust/combustion gas/oil. It isn't common on the B58, they're closed deck, and if BMW feels safe at 240 degree coolant temps (which I think is absurd for the little nylon/glassfiber fittings everywhere) then I wouldn't think you are having head issues.
Another really early indication of head problems is idle/idle offset, and sound- but newer ECUs are so good at managing different conditions that may no longer be as good of an indicator.
Another test is to remove your coolant res cap, (car cold) and run the car for a bit. If the system is pressurizing significantly or has gas escaping before you reach 170 ish degrees, you have a head/combustion chamber issue. They make dye kits as well for detecting that stuff, but typically if it's significant enough to boil coolant you have either a leak/pressure loss, overheating, (water pump/tStat) etc.
