Every business that looks consistently presented is following a small number of rules, not a large one. The rules that matter are the ones a non-designer can apply without asking for help.
Rule one: one colour leads
Choose a single colour that appears on almost everything you publish, and keep it recognisable. Secondary colours support it. When everything is a different colour, nothing is memorable.
Rule two: two typefaces, no more
One for headings, one for body text. Both should be available in the tools your team already uses. The most common cause of an inconsistent look is a font that only one person has installed.
Rule three: agree the image treatment
Decide whether your images are bright or muted, tightly cropped or wide, people-led or product-led. Write it down with two examples each. Image treatment does more for a consistent look than any logo variation.
Rule four: write the tone down with examples
"Friendly and professional" means nothing on its own. Instead, write one sentence in your brand voice and one sentence that is clearly off-brand, side by side. This is the rule people consult most.
Rule five: put the rules where the work happens
A guidelines document stored in a folder nobody opens changes nothing. Keep the rules short enough to fit on one page and pin that page where content actually gets produced.
What this means in practice
If you can hand a new person a single page, two fonts, a colour and three examples, and their first piece of work looks like it belongs, the system is working. Everything beyond that is refinement.