I am getting an absurd amount of hiss and hums
This sounds like settings problems. As Sweetie said, it's a piece of string question, because every piece of gear has it's quirks, two pieces of gear interacting adds more quirks. All this on top of unique challenges every time you move to a new location. My guess would be that you have a low mic output level, which means that you have to really increase the gain on your recorder, adding more preamp noise. There's also the possibility of RF (Radio Frequency) and/or EM (ElectroMagnetic) interference at your location - cell phones, tablets, computers, TVs, fluorescent lights, refrigerators - Pretty much anything that runs on electricity or emits a signal of some sort. Shut 'em all down. This will be especially problematic when using Hi-Z (High Impedance) equipment, which you are; your Hi-Z mic cable is especially vulnerable.
Oh, make sure that you have brand new, fresh, quality batteries. Some budget gear does not react well to low power. Yeah, there's plenty of charge in the battery, but not enough to keep the unit happy. 90 minutes of use can drain most 9-volt batteries to under 8 volts, plenty for quality equipment, could be an issue for budget gear.
Is anybody monitoring the audio when you're shooting? Not you, who as the director is concentrating on the performance, not the technical aspects, someone whose only job is to monitor audio quality?
What you need to do is expand your knowledge base. You can start with:
Gain-staging. This is the process of setting the input and/or output levels on each piece of gear so that you have optimum signal levels with minimal noise. I can't tell you what are the best settings because you will have to discover the quirks for yourself. From there you will find a good basic ballpark setting which you can optimize for each situation.
Proper mic technique. The mic should be above and in front of the talent, aimed at the notch at the base of the throat (where the mic will pick up more chest resonance). If you're not booming your sound will be for crap no matter what you do.
Purchase and read
The Location Sound Bible by Ric Viers. You really need to upgrade your audio knowledge.
Your project will only look as good as it sounds, because
"Sound is half of the experience"
If your film looks terrible but has great sound, people might just think it's your aesthetic.
If your film looks great and has bad sound, people will think you're an amateur.
Sound is the first indicator to the industry that you know what you're doing.
No matter what else you do, GET THE SOUND RIGHT!!!