How did you finagle Diemed anyway? It's "not recommended for players".
Not that it wouldn't be interesting to play, but it was my understanding
that it was meant primarily as an antagonist for Medoere and, to a
lesser extent, Roesone. Also, how do you reconcile playing a Paladin
with running Diemed, which has a history of military aggression?

Landed priest and paladin regents can be extremely powerful, since they
collect regency and gold bars from both law and temple holdings. If I
were your DM, I would make darned sure that you had no chance to get
temple holdings without a massive effort on your part. No doubt Avanil
will greatly resent any effort you make to expand in both areas at once.

If you're planning to take over existing temple holdings, you may find
yourself denied your throne altogether because the priests of Haelyn
could refuse to crown you unless you promise to divest yourself of all
temple holdings...

You're going to need several capable lieutenants as well, since three
actions per domain turn barely cuts if for one type of holding.

My advice: Make a lasting peace with Medoere and/or Roesone extra pronto
quick, since Avanil is likely to become very upset with you....of
course, Avanil would surely offer Medoere one or even two of your
provinces (plus significant temple holdings) if Medoere were to help
remove you from the map. Duene and Tier would make excellent additions
to Medoere. And don't forget that Boeruine would also resent your
rapidly growing power, not to mention Ghoere.