Well specced and great machine in it's day but today it won't cut it for newer codecs, latest versions of Premiere Pro and 4K etc. However it sounds like it does still work well apart from rendering speed so I'm afraid it comes down you if you can accept this over the cost of something more modern.
please tell us your source properties and your sequence settings... I've got a macpro form 2009 (and had to hack it to get it up to high sierra - thanks apple for making this difficult) and it works ok. You might consider a proxy workflow to speed things up. Still will be an extended render to export but can speed things up considerably for editing.