These are good hypotheses. These topics depend on personal taste so I'll include personal experience.
First, let me say that I find the AeroPress manufacturer's (current) instructions to produce excellent coffee -- tastier and more easily than all the other brewing recipes I've tried, including James Hoffman's. Please do try theirs!
1 scoop fine drip grind coffee
1/4 cup 175°F water
stir 10 seconds
to taste: add 3/4 cup water for coffee or 3/4 cup milk for latte
Honestly I do not know why people go to great lengths to do slow brewing, upside down brewing, and other dances. Longer brewing, more water, and/or hotter water are expected to cause bitter "overextraction."
- Their instructions now say to add water up to (1), which is 1/4 cup. This "bypass brewing" approach tastes better to me than their older recipe which said to add water to (2) [1/2 cup], and it tastes a LOT better than pushing all the water [1 cup] through the AeroPress.
- In their FAQ, they point out that the tawny crema might come out bitter. In my experience, AeroPress hot brew has somewhat bitter crema while the (awesome) AeroPress cold brew recipe has crema that I like.
So try this: Press the water through the AeroPress, then press the air into the sink and taste the crema that's comes out through the cap. Keep it if and only if you like it.
- The FAQ also recommends using paper filters because those always won their taste tests over metal filters and they filter out diterpenes which raise your bad cholesterol levels.
About your other hypotheses:
- In my limited experimentation, AeroPress is relatively insensitive to grind size and water temperature (because of the immersion method?). I don't find a scale is needed; I just put 1 level scoop of beans in the grinder at a "fine" grind level or 1 rounded scoop of beans at a "medium" grind size.
- Decaf coffee can taste great. Really! But you have to find beans that you like, just like with caffeinated beans except there are fewer choices. I get good results from small roasters, possibly because they take more care to tune the process and timing for each batch. Trader Joe's and Starbucks "reserve blends" have also been OK.
- I have no experience with LaVazza DEK coffee. The freshness might matter a lot since pre-ground beans are oxidizing faster.
- The Coffee Folk website has a coffee grind chart with photos to help explain. They describe "fine" as "slightly finer than table salt" and "medium" as "most pre-ground coffee" "a consistency similar to sand." Either is probably good, but go with their "fine" size if you can.
- If you can get a grinder and whole beans, that ought to improve the taste a lot. Get a burr grinder. Even an inexpensive one should be OK. There are also hand-operated burr grinders, which are inexpensive, quiet, portable, and better exercise :-)
- I'd really expect pre-ground coffee to have a consistent grinding size. If it looks evenly ground, that's good enough. In contrast, a blade grinder produces widely inconsistent grinds ranging from dust size to whole beans that didn't get cut at all.
- In my experience, decaf does not to be brewed any differently.
Update: You can keep beans (esp. ground beans) fresh longer in a vacuum-sealed container such as a Zwilling "Fresh & Save" bag. An AirScape canister also works but it's bulkier.
So packing coffee gear for a trip means: AeroPress Go, Zwilling vacuum pump and bag, pre-ground beans (or buy the beans while traveling).
Disclosure: I have no material relationship with these companies.