Springs will compress slowly over time but there's really no way to test them other than by ride height appearance. Mine lasted to 200K miles, and were severely sagging when I replaced them -- I was low riding. If you are bottoming out on trivial dips or recesses in the road, then springs should be replaced.
The strut mount inside engine bay is supposed to be rock hard, it provides no road dampening whatsoever -- it merely supports the strut itself.
As stated above, replace the sway bar bushings and the steering damper and you should feel a noticeable difference over small bumps.
You should find out if you purchased cheap aftermarket shocks for your 300E or not, i.e. Monroe or something similar. Bilstein "comfort" are original equipment manufacture on the 300E. Do you know if you may have purchased "sport" or "heavy duty" shocks which would give a much more stiffer ride?
Please let us know what you find out.
'91 300E, 211K miles