when you do the trans oil change I would save some of the oid fluid you remove and send it in for an oil analysis. This way certain wear metals found could point you in the direction of any problems inside the trans.

For my 1998 C230 the owners manual states to have foot on brake when changing from reverse to forward and pause a few second for engagement.

seems like you need diagnostic troubleshooting with the appropriate equipment for the trans and ABS.

also HAYNES offers a manual for your car, U.K. version but helpful.

there should be some old posts about yrans fluid change, have you tried the search feature?