Just from my small sample size of experience, if you have SP only cards, put them in your default rotation, and still set game by game starters. I've found that seems to make HAL think they'll be the next 3 starters, making someone like Hecker a viable option as RP.
As you've found out, a relief appearance after a start isn't pretty.
My first point above may get HAL to use A high priced SP/RP like Hecker out of the pen, if you want to use him as a spot starter and get a lot of RP appearances. If you want to start him every 5th day AND get RP stints in between, HAL gets unhappy about that.
While not in Hecker's league, I use Bennett as a hard LH SP/RP, along with Eggie to spot start and get appearances out of the pen. It works, as I have 3 or 4 SPs in the default rotation, and unlike Hecker, HAL doesn't see them as someone to save for a start in the next series.
Here's an example of what I'm talking about, but the Eggie and Scherman as the RH/LH SP/RP combo. In looking at their game logs, I was able to set them as a per game SP whenever I wanted, but the didn't come out of the pen for 3-4 days after a start.
Hope this helped more than it confused.
https://365.strat-o-matic.com/team/1699874