The main thing you need to do is switch soils. I know mg is easy to get and cheap to buy but it's no good for mj. Yes you can use it and have some good grows, my buddy used it for about year and had ok results but he was never happy with overall quality compared to mine and he switched to Organic Roots and never looked back, I use FOx Farm Ocean Forest and extras.
The main reason for not using it for me is it will create issues you can simply avoid by using quality soil that isn't filled with inconsistent nute amounts. Each bag of mg tends to be different, even amounts within the same bag can be different. I know this through my friend's experience with it, he just had so many issues and in the end is was all about the soil.
Unless I see a specific burn issue or deficiency I recognize and can say with almost certainty I know what the issue and solution is, I couldn't give you advice, and with mg figuring out the issues is as difficult as coming up with a proper solution.
With OR or FFOF it's so simple. You can use full strength or mix in perlite or other soils that are not nuted and if you transplant up to the final pot which everyone but auto growers should do(I transplant twice) then that last pot of fresh soil should last you a good 7-14 days before you would even need to mess with adding most nutes. So by the time veg is done or near done you really haven't had to add in any nutes, just water and maybe some cal-mag type product and root enhancers. So in those soils you don't need to start feeding nutes until late veg or early flower, so it makes it that much easier.
In soil I don't care what anyone here says, you need to monitor pH going in and runoff coming out. I can usually master my soil ph by mid-late veg/early flower to the point where I wouldn't need to check pH runoff any further that grow. You need to know what each of your nutes does to ph, and again, with mg you never know what's left in that soil.
Never switch to 12/12 unless plants are near 100% healthy.
You said it in the end there
Bigger pots(I suggest smart pots or any fabric pots)
Better soil
Knowing pH levels
In my experience tap water will rise if you bubble it, and being most tap water is 7-9 pH using that without treating it would be no good without first adding nutes and pHing or pHing the water only then watering.
When you get a pH meter, get one with TDS/PPM or buy a cheap PPM as well or borrow one. Knowing what ppm your tap water is will tell you a lot. If it's 300 or more you don't want to even use it, you would want a RO or some type of filter.
The beginning is always stressful, you will learn a lot in 6-12 months.
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