The remineralization option is great for drinking water and would probably be fine for the plants too but the water comes out at pH 8 or so and that's not so good. If that part can be switched off so you can just make plain RO water for your plants it would be perfect. We buy RO water for drinking and for my plants as we don't have town water and get our house water from a large dugout on my property. Fine for showering or washing clothes but not fit for human consumption. The pets and chickens get it tho and seems fine for them.
I grind a little pink Himalayan sea salt into my glass of RO water and put some on the top of the coffee when I make a pot too. Tastes a lot better with the salt which is not salty tasting like table salt.
I used RO water with DWC for over 10 years without ever using CalMag and never saw deficiencies but have been using it for the last few years at about 1/4 to 1/2 of the recommended dose. I have always used a bit of epsom salts when growing tho as I used to get a little Mg deficiency. CalMag is way over-used by a lot of people and ends up causing problems. Especially those that use tap water which usually has more than enough Ca and Mg in it that extra CalMag is overkill.
Quality nutes already have Ca and Mg in the proper proportions in them so very little CalMag is needed.