Transmission history, maintenance and how should I proceed?

Some of this has been posted in other threads but to recap..

In May of this year I purchased a 1993 Mercedes Benz 300D 2.5 Turbo Diesel with 197k miles. I had it checked at the local dealer where it had been serviced. Both the owner and the dealership indicated that all service had been performed on the factory schedule. In Aug/Sept the transmission started acting funny which I soon learned was slipping and flaring with odd, hard, slow shifting all over the place. It made me really start wondering if the transmission truly had been serviced recently. I called the service manager at the dealership and pressed him to give me a specific of when the car had been serviced. He dug and found that the transmission had not been service since 130k miles!! The next day I took the car in to get the fluid and filter changed (this shop used normal Castrol ATF) which lead to great improvement, in the range of eliminating 90+% of the issues and greatly reducing the ones that remained. The shop indicated that there was “a lot of junk in there.” Needless to say I was very happy with the results!

So how should I maintenance the transmission going forward?

Should I do another fluid and/or filter change soon?
If I change the fluid soon should I get an analysis does and if so why?
What type of fluid should I use? Note that I live in Phoenix, AZ
Is there any fluid I should NOT use basis on its possible condition?

REMAINING ISSUES:

1) Late to shift into 4th when tranny is cold. Once warm it shifts normal.
2) Occasional hard shifts both up and down shifts.
3) Very rare flares. These seem to be increasing as time passes.

Thoughts?

PaulW

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1993 300D 2.5 Turbo Diesel - 201k miles