Indeed, highly flexible.Bookkeeping alterations are covered in INT (investigation) and helped by forgery kit proficiency per XGtE. A background related to bookkeeping might accomplish the same thing based on using backgrounds for proficiency.
There are rules for creating magic items in XGtE as well, and they do not include rolling to make high quality items. It's a given.
The only thing the soldier background gives that isn't already available is the rank. A person can learn languages and tool proficiencies via downtime already, and a feat for skilled (or prodigy) adds the same skills if they are not already present. Recognizing military rank is something that takes very little time to learn.
A DM determines if the actions a character takes needs a check and what contributes to that check. IME, standard proficiencies plus situational background proficiency is pretty common. If a person wants to "add a background" then the skilled feat covers 3 skills instead of 2 while languages and tools can be learned. Any additional background feature "from a lifetime of..." becomes situation as a boon up to the DM.
Backgrounds are also examples. Make a custom background and use that to make your profession working with your DM.
Which explains why it's reasonable for someone with no training to make untrained checks if the DM warrants it getting back to PC's having seen military ranks and processions at some point as an assumption.
Which gets to another point -- what the profession actually does. INT (history) would cover precedent while CHA (persuasion) would argue the case. Make a lawyer background and use those as the proficiencies. A feature might be similar to the sage in knowing where to find the legal information.
Some of those are examples of checks made without proficiency. Urchin enhances navigating the city while gathering rumors is a CHA check right on the list. I think most DM's just RP those out, however, ime.
Minor correction. DC 22 does exist. Not all DC's go up by 5 such as spell DC's or opposed checks. The difficulty DC's step by 5's.
Mind you, that doesn't change the fact 5e checks work for me. All I need is to have my character perform actions. It works or it doesn't or the DM says roll (x proficiency applies). If I want to soldier or lawyer it's not hard to build something to concept. ;-)