the thing is, those plant substrates dont really do much in the long run...
Actually, that is not entirely correct. People have grown plants entirely in inert substances like pool filter sand including vals, swords, and crypts and I have witnessed this first hand in tanks that I have seen. I also have a cryptocorne in my 5 gallon tank with pool filter sand that is still doing well and growing after 8 months. Plants have to ability to absorb nutrients through the leaf or roots. If the sediment does not have nutrients, they will literally abosorb what they need out of the water column, so as long as you dose the water column with ferts(with high tech, Estimative Index will do the trick), you can still grow plants in inert substances like pool filter sand or 3m Quartz T Grade over the long run without issues. In fact one advantage over such substrates is that they do not mess with you water parameters, so you don't have that initial instability of water parameters that you have to get through and that could cause issues(melting of leaves, fish/shrimp deaths, etc.,). Also keep in mind that substrates like Eco-Complete and Fluorite have nutrients "locked" into the substrate so chances are that unless you go low tech with a good fish stocking level, you would likely still need to dose the water column to some degree as the nutrients will not be freely available to the roots. I am not advocating the use of inert substrates over nutrient rich substrates. Ideally, as Tom Barr points out, your best bet is to have two places from where the plants can get nutrients - substrate and water column. The nutrient rich substrate as he mentions gives you some wiggle room if you get lazy dosing the water column and in a low tech setup it minimizes if not eliminates water column dosing altogether. Pool filter sand is just a good cheap alternative for a beginner who has a big tank and not a lot of money to sink into the hobby.
Mineralized topsoil may be a good option if you have can get all the ingredients you need(or purchase a kit from torpedobarb) and are patient enough to wait out the time it takes to mineralize the soil. Also, at this point, I cannot personally say anything good or bad about mineralized topsoil. When I finally set up the experimental 5 gallon mineralized topsoil tank, I will share by experience and people can take it FWIW. Keep in mind that some people have also run into issues with mineralized topsoil,