Can I ''pre-batch'' my water with nutrients?

It depends on what you are feeding them. Microbes in water will need aeration and without any it will turn anaerobic and not be good for your plants. aka the microbes will use all the finely dissolved oxygen in the water and if you leave that, the microbes will die and then anaerobic bacteria will breed and that normally starts to stink and will damage your roots when you use it to feed your plants. If it is just mineral feeds, they can last a few days, but it's normally best to mix what you need and use it rather than premix.

For whatever reason, most manufacturers recommend using mixed fertilizers within 24 hours of mixing, the exception being those fertilizers made specifically for hydroponics.

Try not to make too much. If the water is clear and doesn't stink, it's generally good enough for a plant.

hey 👋 u can prepare your mixes 2 to 4 days before watering, even 5–6 days for mineral fertilizers, as long as you remember to stir once a day but no more than that. beyond this point, and even faster under poor storage conditions, salts will build up and crystallize. result is poor absorption by the plant, possible root lockout, and in any case some elements will be neutralized or inhibited