Well, my own Star Adventurer was good for about 60-90 seconds with a relatvely light weight 400/5.6 Pentax-A lens. Longer exposures did not make sense because of the periodic error in RA that destroyed to many subs in longer exposures. To fight this I bought an Mgen autoguider. But because the SA is only guidable in RA I was limited by the quality of my polar alignment and had exposure times of max 3 Minutes before the DEC error lead to trails.
I am afraid that 300s exposure without guiding and with an even longer/heavier telescope will not work with the SA so it makes no sense to use Iso 80.
To find best settings for a K5 I would test what is the longest exposure without trails (not those trails caused by periodic error) at high Iso and then gradually turn down the ISO until the sky background peak is at about 10-20% in the histogram. And then shoot as many subs as the night offers you to get a good base to use the best at least maybe 50 of them.