In my humble opinion - there is no legal way to use as in the 'bicycle' and full safe way to get this motor in EU.
Well, you may be right in that some officers may choose a stricter policy than the law itself... but the letter of the law makes no such distinction.
Set the speed & wattage limits, under-rate the motor, label it as such, & you've met the legal requirement.
Some officers may disagree &
choose to harass you about it, but
the law itself is pretty clear: There's no requirement that you be "big" whatsoever.
Also to clarify: "Big" prebuilt bike maker's "250 Watt" EMTBs are already capable of higher nominal Wattage, & are merely set to a limit lower than they can handle, for legal reasons. There is no restriction against this. Under-rating equipment is routine, for the automotive & motor industries; it has been going on for decades, due to regulatory & insurance incentives.
Reducing a motor's nominal Wattage rating below what it can actually handle, is not just acceptable,
it's the norm.
As the small player you need to have motor with the papers. Its no matter what will be inside, what firmware you will put to it or what physical modification you will do. You need have the 250W rated (by someone bigger) model enclosure with correct stickers.

You can choose to believe that if you want, but there's exactly no part of the law which states that.
There is no "someone bigger" required, to meet the legal requirements for reducing the rating of a motor.
There is no legal restriction on who the manufacturer must be, & reducing your rating of a component for safety & liability reasons, is both legal, & widespread industry practice.
The "papers" needed are literally just a stamp you made, with the relevant article of law cited, affixed to the motor.
It
isn't complicated, or difficult, & personally I believe the only reason common perception differs, is because some bike makers & a few displeased officers, would prefer that the law be more restrictive than it is, so they spread FUD.