I'm trying to create a homebrew monk subclass, with the idea of being a more strength based rather than wisdom. I want to use strength in place of wisdom for monk abilities, like unarmored defense and the Ki save DC. I'm not sure how, if it's even possible, to change those, or if there is some kind of work around for this (such as creating a subclass feature that can make the Ki save DC = 8 + proficiency bonus + your Strength modifier, and same for unarmored defense) but I'm not quite sure how to add those properties into the class feature I create. Any help would be greatly appreciated.

You can add a new AC calculation fairly easily - that wouldn't be a problem. But changing the Ki save DC - I believe - isn't possible because it's a base class feature and those cannot be modified with the current homebrew tools.

You could add a feature that literally says "With this subclass your Ki save DC is ##." but it wouldn't change Stunning Strike (for example) to use that value. It would just be a note that the players of that subclass would have to remember.

Thanks for the input. I'll keep in mind about keeping it as a note. I was hoping there was a way to include it in the character sheet calculations, since it defaults to the normal one, but I'll just have to use my brain I suppose lol. Regarding the AC calculation, how would I do that? I am so new and unfamiliar with D&D Beyond (I know it says I joined 2019, but i made it and then never touched it for the last two years) and have kinda just been trying to self teach myself on it.

*Edit: I was able to add a modifier to the class feature to add my str into AC, but doing so still adds my wis mod instead of replacing it since that's part of the Monk unarmored defense. Is there a way to adjust that as well? Or some way to make a work around?

The easiest way to figure out how to do anything in homebrew is cheat. Find something official that already does what you need and make a duplicate copy of it - then go inside and find out how it does what it does.

In the case of making a new AC calculation - I think (I've never actually done this) it's:
Modifier Type: Set
Modifier Subtype: Unarmored Armor Class
Ability Score: Strength

Thank you so much, that worked perfectly. The way I did it before was as a bonus, so it basically made my AC Str+Wis+Dex. They way you suggested made it so its Dex+ Str or Wis (whichever was higher). Thank you so much for the help!