In an earlier career, I got certified as a Microsoft Office Specialist and did a good bit of consulting with corporations whose staff were in need of training on Word, Excel, and PowerPoint. That certification has long ago expired, but I can offer some help here.
Over time, I have built up a large custom dictionary of words that I use often, just by adding them to Autocorrect the first time Word flags them (or manually, as explained below).
When Word insists on what I consider "wrong" corrections, I open File > Options > Proofing > Autocorrect Options. Then I search for the offending word in the Replace text as you type table at the bottom (for example "red => redundancy") and simply delete that correction. While you're there, you can manually add any autocorrections that you like. Be aware that any changes made there apply every time you open Word or any other Office product, so if it's a one-off, it's best to correct it manually.
For example, if I have a character named Frederick and am tired of typing the whole name repeatedly, I manually create an autocorrect to replace the characters "fr" with the full spelling of Frederick. Then, I just type fr, hit space or any punctuation mark, and Frederick appears. If there's already a replacement for fr in your dictionary, simply retype what you want it to replace it with.
There's another way to set up your custom dictionary, but I recommend caution using it: you can directly edit the RoamingCustom.dic file using Notepad. I might do this if I had a large number of unusual words (such as words in Russian that I used in the last story in my Wearing Green on Thursday series), then I might type them directly (including the Cyrillic characters) into that list. If you try this, be sure you make a backup copy of the file before you make any changes! And if you actually do hose it up, you can always restore the default dictionary by going to your App settings and clicking Repair. You will, however, lose all customizations.
Also in Autocorrect Options, I set up AutoFormat As You Type and AutoFormat with the choices I prefer. You can tweak these all you want until you get it the way you like.
Finally, I open Writing Style: and Settings at the bottom of the window and turn off a lot of the so-called grammar checker options. The free version of Grammarly is a far better tool, since it's specifically designed for this purpose.
This is probably TDMI, but I hope it helps. 🤩