aside from your bulk substrates not pasteurizing properly due to what sounds like improper pasteurization, you can avoid contams, increase colonization times, and produce better results in general by using a higher percentage of spawn. It is always a race against the badies so the more inoculation points mixed into the substrate, the faster you will finish the race...
I use bags extensively with bulk substrates but unless you solve your pasteurization woes, you are going to have the same issues you were having with tubs or whatever receptacle you have been using.
Im not sure what methods you have been using to prep your spawn and I have never used an insta-pot but it sounds like you are not keeping your sub at pasteurization temp long enough. I also make sure my substrate is wet the whole way through before starting my steam pasteurization. I assume the substrate is submerged entirely in water when using the insta-pot but I am not really sure how those things work.
How long are you pasteurizing? I would increase your pasteurization times.
I would suggest soaking your straw overnight before mixing it with the rest of your substrate so that seeds and endospores germinate. They are more easily neutralized once they have germinated.
Is your substrate fully submerged in the insta-pot? If the top of the substrate is not submerged, it will not pasteurize properly.