Way I see it, for FFP to really, genuinely work they have to massively increase the clarity and transparency of the system.
First, they have to be very clear and public about what revenues can be included by clubs - for me that should be match day income, TV money, commercial revenue, sponsorship and
gifts from owners. Not loans. If an owner wants to put his (or her) money into their club to give it a boost, fine, but they should not be allowed to saddle the club up with debt in the hope that they can get to the Premiership one day and pay it off.
Then every club should have to publically disclose those revenues, and make any paper trails available for the EFL to audit.
You then have an agreed percentage of the previous year's revenues that any club is allowed to spend on wages in the next (maybe 60%). And then the clubs need to publicly release those wage figures as well to show openly that they are compliant.
Yes, players and chairmen alike probably won't like it, because that sort of transparency shows up to the fans very clearly who is, and who isn't value for money.
But it's done elsewhere. I've posted it before on here, but here's the wage bill for the hockey team I follow -
https://www.capfriendly.com/teams/canucks
And here's one for the baseball team -
https://www.baseball-reference.com/teams/PHI/philadelphia-phillies-salaries-and-contracts.shtml
The NHL has a salary cap, and MLB does not, but the principle is the same. Openness and transparency, and then it becomes much harder to abuse the system.
If they can do that in North America, why can't they do it in Britain?
But if the clubs won't sign up to openness and transparency, and want to continue to operate FFP as a shady, opaque system that mostly ends up in the courts to resolve, then it isn't going to work. Then you have to go for a simple, hard salary cap as it's the only way to create something enforceable.