Bwahahaha too funny you 2 dirt bunnies
Dantra that's a great thread you linked
but a lot of effort for roughly the same results.
Do you ever do any tests on you're water? pH, ammonia, nitrite, nitrate, hardness testing of any kind?
How well does the java moss, java fern and hornwort grow for you?
Getting a bigger tank are you changing the lighting?
(I realize I'm answering you're posted questions mostly with my own.)
Regarding how long the soil base lasts will depend on the nutrient content starting out, the organic content to help sustain it long term and the nutrient demand placed on it. That demand is driven by how much light energy is provided the tank more than almost anything else.
With my belief that light energy is easiest to understand and adjust without changing the tank in any major way. Light is the dogs head and everything else is just the stuff between it and the tail. Light drives demand. So I asked about intended lighting.
All my new tank starts now include floaters. Added starting out many are cleared completely after a few months.
Floaters are a great help controlling light rather than changing photo period as another option. Filtering the water too goes almost without saying. Removing free nutrients from the water it all adds up to floaters helping a great deal to avoid algae on a new tank. Ricca, Frogbit, Moss or Subwassertang, Salvinia cheap easy plants with great value (imo).
That said having a tank set up with bagged dirt alone MGOCPM (potting mix) and nothing else for over a year I saw nothing to indicate the plants were lacking any nutrient at all and growth never stumbled. Beyond 3yrs. now and still no additives or changes made. Growth has slowed but the tank is still stable and growing plants.
Flourite, Eco Complete and larger gravel mixes allow the soil to release more to the water during the first few months is my experience. Using sand or a mixture of sand and smaller frag my water parameter always test a lower nutrient content. (Web searches should pull photo's of different substrates for reference sizing.)
I don't care for traditional river gravel very much as it doesn't produce a very good boundary layer. The round pebbles do little to hold the dirt. Even in a thick layer (6cm or more) traditional river gravel still allows some soil to work out into the tank water.
welcome to our dirty world
(but I think we should answer a bunch of questions both ways to help)