This is a job better left for others...
Yes, I've been down this road. I tried everything you have tried and it is futile without the proper MB plate compressor -- especially if your springs are sagging.
You'd be better off paying Pep Boys to install the new shims for the $238 they charged me to replace all four of my springs.
Don't ask me why I chose them -- I'm sure any undercarriage shop could do the work. It's money well spent -- one less headache and possible hole in the head if a spring decides to shoot out.
'91 300E, 209k miles