In the drop down including on a purchase invoice for FA posting type, there is only currently "Acquisition, maintenance & Appreciation" I suggest add in "service" which will pick up the document date or posting date (customisable by company).

Add to a new field " Last service date", then have a date calculation option per FA as this is a variable, and the then use that to determine "Next service date" which will then show on the Maintenance - Next service report, as the current manual updating of the next service is too manual to be consistent.