You also could try rEFInd:
https://www.rodsbooks.com/refind/
See... this is why I need to read the Whole O.P. before I rush to answer...
Here is a Full-Length guide that you can peruse to see if it includes any steps you might have missed. This may be a more direct option than guessing in the dark:
https://www.makeuseof.com/tag/install-linux-macbook-pro/
You might consider a Fresh Start while following the guide as well.