Several thoughts:
-you're right, staying with Alaska doesn't make a great deal of sense as it sounds like you'll rarely actually fly them. OTOH, if you really dislike Skymiles and are convinced you want to continue flying on DL, you'll get some bennies on the SEA - MEM flights by sticking with AS status (free bags, priority seats, occasional upgrades), and maintain better award options of AS miles.

-are there other options for your SEA - MEM flights besides delta that are reasonable? If so, you could consider that airline (UA, AA); if not, as above, you're stuck with DL or AS

-FWIW, I hear some FTers claiming that Delta treats their elites very well; the major dissatisfaction is the award redemptions for saver space anywhere. If you ended up diamond on DL, there's a reasonable chance you'd see pretty good upgrade percentages on your SEA - MEM flights.