RE: Splinterlands - I decided to hold a minimum of 1 million DEC for long term

I do assume that you are using an automated renting service to manage this all. If not, you could increase you earnings with one of those.
I am using https://splex.gg/?ref=zeqxyzex9bftxid4 which makes sure that almost 90% of my rentals are actually rented out.
Feel free to use the referral link, if not you are smart enough to know the site.