No I think lots of low tech lush tanks exist, but I'm saying your tank was mine for years and only those changes got me what I wanted.
This pic below is zero tech planting but its easy for a five gallon sphere.
To load your substrate up with enough organic decay will take a long time
The organic decay in this sphere is natural co2 pump, can run fifty years or more easy, and it is the ferts. All I do is feed shrimp and change water.
But getting a really clean, slightly eutrophic 90 gallon up like that will take nearly a substrate replacement so I'd work another angle.
I know you dirt loaded it but that sag isn't being nourished either way. I don't see detritus poking up from that quartz, perhaps the roots are mostly resting in inert substrate, systemic nitrogen is low as reported, co2 etc not maximized thats my call.
You can cheat that by dosing light ferts with co2 its steroids.
Contrast the co2 injection and uv, the two chief things you could do as an upgrade, with this bowl that uses only the densely organic substrate approach.
This bowl is non heated non filtered shrimp bog its nearing 13 years
Why no chlorosis with no tech, no co2 and no filters? Totally different design.
Not inert quartz (which isn't bad when enriched, its a nice grain size river bed look you just need to charge it up)
The bowl focuses on having substrate with 13 years snails and guppy waste loaded plant specific laterite packed flourite substrate
So it doesn't need high tech but i do drive a corvette light on this thing. A secret to low tech for me is to not skimp on lighting. This is 14 k old school 150 mh lighting, what you wanted in 2004
You won't see me on led this mh plant light is sick. So my point is, each system has a specific design to address plants getting nutrients and since we aren't working from the start to engineer a high organic substrate, don't go that way go ahead and max out the tech with a co2 setup they are really pnp nowadays and some uv your system will change.
Just the uv alone could zap all that algae, it did mine. I used a uv for a five thousand gallon pond it wasn't all that big. All new bulbs, Dupla co2 all in one kit, fat uv, you love your tank in four mos.