Hi @isaakgilissen
Try replacing the drive belt.
Search online for ''Sony 3-377-420-11'' to find suppliers that suit you best.
Also use some Isopropyl Alcohol 99%+ (available from electronics parts stores) to clean the capstan, pinch roller and the record, playback and erase heads in case they're creating a drag on the tape.
Here's the [https://www.manualslib.com/manual/1197727/Sony-M-430.html#manual|service manual] that may help