If you don't want to time-faff the party, you will need at least three encounters out at all times. One will be currently interacting with the party, one will be set up and ready to go, and one will be being briefed (the other monster statter will be moving on to the next encounter site to spot obstacles, dump the kit somewhere, and make ready to run back and collect the monsters from the currently interacting encounter so they can start briefing them on the way up to the encounter site). This means that your encounters are generally in ones, twos and threes, with four and five (i.e. equal or almost equal to the party numbers) being 'big encounters' that you have to put one-monster encounters before and after.
Final Encounter Syndrome arises because the only place you can afford to deploy all or almost all of your monsters at once is at the end of the linear when you don't need to hold some back for the next encounter to be set up and ready, so it's the only time you actually get to outnumber the party with monsters. Given that most monsters in the system at the moment seem to be defined to be less hard than most parties (they tend to be in the 90-150XP range if that applies, or roughly the same power level if that doesn't), outnumbering the party is the only serious way to provide a combat challenge.
So, there are some trade-offs here:
1) If you want bigger non-final encounter sizes, you end up putting out only two encounters at a time or a string of one-monster encounters that the party often just kills out of hand / runs past / adopts. Then you time-faff the party. There is a trade-off between encounter size and likelihood of a time faff.
2) If you want more balanced combat challenges throughout the linear, you need to up-power the monsters significantly, *and give them status effect immunities*. Because three monsters lose to two mages if the mages are Crowd Control spec and any good.