No suggestion of sterilizing your filter
As I mention, it is possible to move all those fish to temporary holding tanks but it's also possible to give the tank a good clean with fish in situ - just much slower process & usually more stressful to fish.
If you disturb substrate with fish in tank, it's conservative to have done extra water changes leading up to this maintenance, also dose Prime at intervals through the process (it will help bind various released compouds), perform a large water change immediately afterwards.
In a tank this size (& sounds heavily stocked with some rather expensive fish), I'd only deep clean 12 - 18 inch (length) by 24 inch (tank front to back) "strip" of substrate each "clean day", and give fish at least a day between "clean days"
Discus being discus, I'd still remove them to temporary holdings (sponge filters & 90% daily water changes works for breeders - do some research on this for details ... sorry I don't recall how discus experience you are) & return them to the tank once all the messy cleaning is done & tank has settled.
There are many many posts on this forum about BBA & various methods of limiting it's growth, dealing with excess etc
BBA doesn't happen overnight but is a fairly slow invasive process, & during that time, conditions must be such that BBA is advantaged & plants disadvantaged - problem is, that balance point in a planted tank is not so easy to recognise or maintain ...
I don't see any mention in your linked thread of CO2 levels or water changes or plant maintenance routine or light schedule or ....
- perhaps if I scrutinized each post I might glean that information but it seems reasonable to me that you would provide those details if you're interested in suggestions
Once it's in the tank is it game over???
No
as soon as I start planting it's going to come right back?
No
Am I best of waiting a few month
Sure - set up tank as a Discus biotope in the meantime (well sort of biotope
there are no cardinals & rummy noses anywhere near wild discus habitats)
It's an excellent time of year to get plant propagators going, you can buy just a few pots of sterile tissue culture plants & grow out a forest to restock your tank with
There are some excellent propagator threads & lots interspersed into various tank journals
Tank was looking grand before the Invasion - well done