Thank you for sharing your experience and lessons learned. I will probably try a dirted tank in a 5 gallon some day.
In our 20 and 65 gallons, we went with something odd, I think...
1 inch bottom layer composed of hardwood pellets (for smoking meats) with diy N-P-K root tabs sprinkled alternately with a few diy iron root tabs and seed material like organic compost or gunk from a seeded filter, then capped with 1 inch coarse sand. The idea behind using the wood pellets is similar to heugelkultur. When we reach the 1 year mark, I will report on the plants, but barring another water quality issue and ensuing, panicked beginner hobbyist response last year, the plants seem to be doing okay right now.
So, it is basically an inert substrate. I expect to replenish the root tabs periodically anyway, so it won't be a hardship to do this later on with a future dirted tank. I mean, when the soil layer gets used up, I don't intend to redo the substrate. Sounds like a pain! I will just use root tabs.
Just like your observation about the soil later needing to be waterlogged before capping, we had to do the same with the wood layer. Otherwise, lots of bits and pieces floated up.
Coarse sand and calcined clay (Safe-T-Sorb, on a separate part of my 65g) seemed easy to plant in compared to gravel in another tank. Now, if only I can get the jumbo amano shrimp and mystery snail to stop pulling the stems out. I like the idea of mounding up or surrounding the stems with small rocks.
I saved a little money by using Black Diamond blasting sand mixed in with the coarse sand.
I saved the large water changes for after a 4 week period of ghost feeding and keeping the tank covered to keep algae from getting a toehold. I put the plants in after the 4 weeks.