Thanks!
I'll try that.
I mixed 4.5 ml of flavour (100% PG), 10 ml of 18mg nic (50-50%), 10 ml of VG and 5.5 ml of PG (all in that order). I use a plastic 30ml Unicorn bottle
Apart of a couple of initial warm baths, I left the bottle open for about 30 min once and reopen for a few minutes everyday. Maybe that oxidised the nicotine? Also, everyday did a bit of gently shaking the bottle. Maybe I was taking too much care of my liquids
waterbaths are used to thin down/out thicker juices aka high VG, because it takes already longer to get end results from higher VG juices in comparison to thinner juices. In your case 50/50 this doesn't apply and is not necessary.
Also how warm is warm, I see plenty of people using warm/hot water that comes out of there fausset, thinking its safe. Well in most cases it is not, taking a thermostat and actually measuring it, youll be surprised how hot that can be, which leads to the problem of messing up the nicotine.
Breathing is another well known method you're mentioning, and unless you using anything alcohol based or extremely odor intense flavoring, there's no reason to do that either at the start. You can do it but after a week or two till the actual flavor profile settled or as I mentioned you using a alcohol based flavor a lot of shake and vapes mostly do, to get that out you would open the bottle for 5 or 10 minutes not longer.
Mixing 10ml in a 30ml gives a lot of space, and is great when shaking it, but now giving it that much space for air that you let in with the breathing method, well I'm not surprised its harsh or tastes off lol. Also if you doing the water bath as explained above, never do it in plastic bottles, it don't matter if its high end or cheap, it will interfere with the results. Get 15-20ml glass and put it in there, shake it, put it away in the cabinet and wait.
I'm not sure which milkshake concentrate you bought and company you got it from, regardless tho all of them need a minimum of 3 days up to 7 days of steep, some even longer. Most of these are also at least 70/30 ratio, because while you can get a milkshake at 50/50, it wont be very creamy/authentic, its more like the skinny cow version of a milkshake. But i guess thats preference, it would help alot with the harshness so, after you stopped overheating your juice and airing for 30 minutes lol. You won't taste anything prior or it will taste very fruity, some taste very milky at first and so on. I could go on, but I think the point I'm trying to make came across lol.