Style guides, for editors and translators, keep us on this side of mentally healthy. I am an American but do a lot of editing of Australians, Canadians, British.... the style guide makes it so you aren't changing "different to" to "different from"-- if your proofreaders are professional they should know that.
Chicago and APA are the most common in my areas (science/tech) but they are very America-focused, and Chicago Manual of Style has a cost barrier (you will be looking at it all the time, so you will have to pay the subscription fee). Chicago does require the Oxford comma, as does Oxford UP style and Strunk & White`s Elements of Style (another old but good reference).
I have colleagues who work with the Canadian government one and their comment is that it is hard to keep up (not always consistent the way you might expect things to go. That said, any style guide is always changing.) There is also the Canadian Press manual https://www.thecanadianpress.com/writing-guide/
. Some others can be found here, including one from UBC. https://www.noslangues-ourlanguages.gc.ca/en/ressources-resources/ressources-resources/guide-guidelines-eng
Last but not least there is always the Economist`s style guide. Even though it is British English I use it with some clients who would generally probably prefer American English. It is very clean, succinct, and easy to apply. It used to be available for free but now is an actual book for sale. Review here. https://www.editage.com/insights/the-economist-style-guide-10th-edition
A style guide should save you headaches by reducing the number of things you dither about and making things more consistent (as you said, it is maddening to see 5th and fifth in a text, for example), but there may still be things you are going to have to decide for yourself. Even the Chicago Manual, which has to be a good 600 pages, doesn`t cover every case and I have had to debate with colleagues about how to address exceptions in a journal.
Finally, there will always be more errors. You can have 8 proofreaders or 100, there will still be something that escapes. We all do the best we can, but we`re human (and trust me, the bots screw it up MUCH worse than the humans do). That`s why books are published in different editions, with errata. Things slip past, we try to minimize them using tools, but it happens and you shouldn`t be too concerned. And another note about that "teh"-- when I see that in a text I edit I realize the person doesn`t have their word processor set to check their spelling correctly. I use editing software that helps me catch that kind of stuff as well as consistency, formatting issues (one tab or two before section heading?) and recommend it highly. If you work with Word, you can purchase PerfectIt, which fits in there and can be set to any style guide you work with. There is a serious learning curve, but if you have a text in Word (or a word compatible suite like Open Office, etc) I would be happy to run it through the software for you when you get to that point.
Sorry for the tome. Catch me before breakfast and I`m a bit wordy.