The OP asked: Does this mean that modestly stepping aside is a very good practice ...?
On the other hand, Walser’s celebration of the monotonous or uniform
returns us to his fascination with subservience, with relinquishing
all personality to imposed order: “Modestly stepping aside can never
be recommended as a continual practice in strong enough terms.”
According to Walser, "to step aside" is recommendable and perhaps considered even an act of courtesy and modesty. I understand it to mean you should not be avaricious to appear in the limelight; to be a protagonist; to be seen as a strong individual or a leader but allow the higher ranking person to appear on centre stage, to take command, as it were; to guide and lead the way. Moreover, this type of behaviour should not be limited to the occasional or spasmodic episode but rather it ought to be
a continual practice.
Subservience means excessive obedience, submissiveness and servility. All traits which in today's modern society tend to be discouraged and frowned upon, however; Walser cannot emphasize just how important it is to remain silently in the background, to negate one's identity and merge in with the rest. This is what is understood by the expression
in strong enough terms
Finally, in answer to the OP's last question:
If someone says:
I cannot thank you enough he is saying that no matter how grateful she/he is,
how often she/he thanks us personally, it will never be sufficient/enough.