I’ve used masa harina a lot for past recipes. It is cooked already as it is nixtamalized, so no worries there. Some people will be bothered by the grittiness, others won’t, I had no issues with it personally.
The brand can make a difference as well. Many seem to like the Maseca brand, I’ve used it and found it good, I found the smoothest to be Masa Brosa from Walmart, and the roughest to be blue masa.
Many folks like oats and use them without any pretreatment, but from what I understand you can easily cook them with a short microwaving.
As to overdoses, just be careful with them and do your research on the nutrient of interest. Some are disputed, or known to be significantly exceeded in some medications and would likely be OK. Others, the form it is ingested in can make a big difference in absorption, so check the form of vitamin you are getting as well.
There are some nutrients not specifically listed you may also want to include, or you may want to be sure to get a specific form of a certain nutrient such as D3 or K2. There are also ideal ratios of some nutrients to strive for, but I wouldn’t be to concerned about those when first starting out, but definitely worth looking into as you advance as a DIY’er.