What could cause lack of growth?
Million of things
As you know this is very complex and even when you are still doing fine, there may be plenty of things still going wrong.
I would really like to see some improvement in the current tank before investing time/$$$ to a complete restart!
Yup, but sometimes a little investment helps to avoid frustration. Additionally, melting plants, trying new equipment, etc. is not cheap... you just need to get rid of the substrate, give the tank a good cleaning and buy a new substrate and plants. No need to buy new equipment. My larger tank is 60 cm, this make a potential reboot or new layout very cheap compared to larger tanks.
But this is kind of contradictory to your advice anyway!
Not sure... it looks like most of the things you do now are right but IME miraculous recovers are difficult and probably restricted to knowledgeable guys or with tanks that suffered a disaster but were resilient enough. I.e. my sister in law was in charge of my 25 liters tanks during 10 days this summer. She had very accurate instructions on how to top off the tank and how to apply ferts, all this every four days. Amazingly (not only to me, but to everyone that has read the directions) she misinterpreted the information and topped off the tank with ferts! During 24-48h the tank had at least 150x the EI nutrients level. All my shrimps died, all my Rotala red background melted and the rest of the plants (H. pinnatifida, several mosses, Anubias, Bolbitis) survived. There were some spots of BBA, a bit of BGA and GSA... One month later the tank is doing very well. BUT this was a very short stress period and the tank was very healthy before this. Intense WC regime and severe pruning was all I have done. My first tanks never suffered such important disasters, but I had melting and algae problems for one year before I was able to manage them properly... and even if my tank management/maintenance was exactly the same and I was using the same equipment I am using right now, I never achieved such health tanks as I have now. When you will dismantle your tank you will be completely amazed of how much dirt your substrate is holding, even if you siphon it... it's really unbelievable.
Anyway, I may be wrong. Not sure if it is your substrate with plenty of bad stuff or just that your tank is not resilient enough (= very weak from the biological point of view: low plant biomass, low bacterial colony, etc.) to face the minimum problem that arises... Build a solid structure from the beginning and you will get high
Jordi