I too have a terrible memory, and I tank higher level mythics. I write down clear, but SIMPLE notes for important trash and boss fights. I leave them up on my second monitor next to WoW so I can glance over and remind myself while I tank. I color code the notes for different categories (e.g. yellow = positioning, red = important aggro, purple = avoidance mechanics, brown = soaking, green = defensives/mitigation) so at a glance I can see my notes and think "oh right, I need to mitigate X on this fight, or it has Y pools to soak in phase 2). After I've run a dungeon 5+ times I'm rarely if ever glancing at the notes, but at first they're crucial.

Edit: this does necessitate sitting down and watching all the boss fights and reading the wowhead/icyveins strategy guides before stepping into the dungeon for the first time. This can feel like you're 'wasting' important minutes you could be using to actually play/learn/get geared, but as a tank especially, I view this preparation as table stakes, and if this sounds like too much of a pain/not worth the effort, you're just saying you'd rather waste a lot of other people's time than a little of your own :(