In my opinion, you should go full DET with enough PIE. What constitutes of "enough" depends on yourself, but if you are not doing FCoB, having a comfortably big MP-pool won't be a big issue. So that being said, if you intend to do content casually on your WHM and are working on upgrades simply to pass time, you don't really need much if at all PIE, meaning you could go DET+SPSP like you planned to.

If you intend to do some (semi-)serious content on WHM, I'd say: enough PIE + full-on DET if possible. 5k to 5.4k MP (with food and party buffs) seem about right, with some going for even more.