Often in the US "cross XXX at or above AAA, cleared for the approach". Takes the guesswork out of it.

I think you should.

"Must" sounds like a lawyer's term and perhaps such exact rule, spelt out clear, will be impossible to find. An instruction to descend is exactly that and level flight does not fit that description.

An argument about a need to slow down for configuration change in level flight
- is a strawman since it can be done differently
- if it is short and temporary would not be a problem anyway.

Your example, in the opening post is benign. Let's blow it up while keeping it real:

- by ARINC definition the final app angle must extend in front of the FAF/FAP to enable capture from below, once established on the final track
- on many installations, this will create a pseudo G/S intercept quite far out, maybe from 5.5, 7000 feet (platform ALT 3000 let's say)

Once after the final vector, with clearance to descend 3000 and for the approach, one may wonder if maintaining the 6000 until the 3° is all right with everyone.

I think it is not, unless coordinated.

All approaches have FAF altitude or platform altitude. Generally the clearance is given to that altitude. Sometimes when the clearance is below that you should descend to that after confirming.The approach profile can be intercepted below FAF altitude without modification. In case of managed NPA it's not permitted to modify the FAF altitude.

No. You do not need to descend at 500fpm. You can descend at 50fpm if you're that worried about flying level.

If You are talking about CDA :

no segment of level flight longer than 2.5 nautical miles (nm) occurs below 6000ftQNH and ‘level flight’ is interpreted as any segment of flight having a height change of not more than 50ft over a track distance of 2 nm or more.

Hence, even 100 ft/min will work at typical terminal speeds.

Last time I did the calculation (do not ask me why:p) 100 fpm was enough, may have been a different airport.

Regardless, easy really does it. Follow the instructions honestly, speak up quickly if you cannot, and discard all bright ideas for great solutions out of the orderly flow.

Short version:

https://cimg8.ibsrv.net/gimg/pprune....2ccedf354.jpeg
This is for a specific aerodrome, what are the actual general rules? I’m not talking about “I’m not really sure so I just fly 50fpm so I can’t get pinged either way”
https://cimg4.ibsrv.net/gimg/pprune....8515aaaf1.jpeg
This is the scenario I originally described - we are saying this cannot be flown as coded? (Not talking about any modifications beyond FAF)

I think I know the situation the OP is referring to where an ALT CST* of 4000 is coded into the STAR but is not actually required when approaching the ILS from the east. If a pilot continues in managed descent then the aircraft will level off at the CST prior to intercepting the G/S instead of continuing the descent to 3000' which is the cleared level. In my view if ATC have cleared you to a specific level then that's the level you should descend to. They are not aware of how the FMS is coded and are not expecting you to level off. If you need to slow down to configure then select V/S so that the descent to the cleared level is maintained.

As ATC use performance as one of the methods for standard separation, then if there is an aircraft performance requirement on the day to not perform in the anticipated manner, notify ATC, or agree with ATC a different course of action.
Ensure communication occurs. CRM includes ATC. Share the “mental model”.........one of the competencies of multi crew flying....l