I have maxed out 3 of the areas for DOFFs. Is there an end-game for the DOFF missions? If we max out all the fields... is that it or is there something planned for this?

There is another Tier coming with Season 6, and as for "end-game" with DOFFS, completing the sections if far from it.

There are many chains to complete, or there's converting all your DOFF's into purple in the same way you do your gear....that's a lot of work even if you have every section at 150k cxp