Hi @kylecage0 ...
Have you explored DesignPLUS from Cidi Labs? If not, I'd highly recommend that you do this! DesignPLUS is a paid add-on to Canvas, and you wouldn't need to keep hard-coding HTML files, uploading them to your server, and then embedding them onto Canvas pages using iframes. (Gosh, that sounds like a lot of work!) DesignPLUS is really powerful in that it helps you design course pages without needing to know any code...as most everything in it is WYSIWYG (What You See Is What You Get). You should also check out their Showcase Page to see examples of what different schools have designed for their own course pages.
DesignPLUS lets you build banners, content blocks, and other elements to design full course content pages within the Canvas RCE (Rich Content Editor). From a Canvas administrator perspective, you can create all kinds of content within what they call a "Primary Customizations Course". This course is where admins can add things like content blocks, banners, link grids, and templates that all instructors in the Canvas account would be able to bring into their own course content pages. So, it's entirely possible to design templates and content pages that have a similar look and feel...or that have some common elements that all courses should have, and then instructors can bring that information into their own courses.
In addition to the main primary customizations course, you can have a customizations course at each of your sub-account levels. This means that if you had content that you wanted only an "Accounting" sub-account (for example) to have access to, you could build that content into the "Accounting" customizations course, and it would only be available for those instructors who have access to courses in that sub-account.
There's just so much to explain...and really too much to provide in detail here, but I'd encourage you to check out the links that I've provided above. Also, I'd encourage you to watch some of the videos here: Webinars | Cidi Labs.
With your Canvas Commons idea...if you had a course where you had shared a page to Commons and then people imported that page into their own Canvas courses, you could update that page in your own course. Then, you could re-share that page to Canvas Commons. Then, other people would go to Canvas Commons and see a message stating that a page had been updated...and prompted if they wanted to apply the updated page to their own course(s). It's kind of an "update once, distribute to many" approach.
Anyway...I hope some of this information will be helpful to you as you explore your options!