Can I mix dry ferts to make liquid and then dose an amount that doesn't require 50% weekly changes? I am hearing the only way algae grows is from not enough ferts (not from too much, hence the water changes).
Yes, in fact you don't even need to mix solutions - just dose smaller amounts of the individual salts. EI was created as the "easy" way of dosing a planted tank, you can do it the old fashioned way by adding only what the plants can consume.
To figure out exactly how much the plants are using you can either observe growth or test the water.
Say you have a nitrate test kit, start by dosing say half of EI and measure nitrate daily. If the nitrate level is rapidly returning to zero before the next dose you can increase fertilization slightly. If nitrate is constantly rising, pull back a little.
If you only have the one test kit, dose the other two salts proportionally to the nitrate. If you have a lot of spare time you can perform a similar process with the phosphate/iron test kits and dose all three salts individually.
Once you have things in line you can reduce test frequency. Note that sometimes nutrient consumption will change (especially if bioload increases) so I'd probably run another set of tests every month or so.
In general I've found that each individual aspect of the planted tank (light, CO2, NO3, K, PO4, Traces...) has a very wide margin of error, you only really run into trouble when many are out of balance or one is at an extreme.
The hardest part is when the tank is just getting started, especially if all your plants have been weakened by shipping. My best recommendation is to get at least one bunch of plants locally - find the healthiest looking fastest growing weed possible (floaters are even better) and everything will be great. I started a tank once with only shipped plants and they spent the first three days shedding leaves which made things tough.