UPDATE: I figured it out myself!
After literally three weeks of hopeless googling and tinkering, I turned to the wordpress forum. However, after a lack of response, I returned to searching. More. And more. And again. And more.
Well, hard work DOES pay off (though the frustrations were endless, it felt).
For anyone suffering this issue, you have to add a little code in your functions.php file. I found the solution here: https://www.wpbeginner.com/wp-tutorials/how-to-add-custom-post-types-to-your-main-wordpress-rss-feed/
Now, my regular feed address is working like a CHARM! Embarrassing how long it took to find a solution, but hopefully anyone else suffering this same problem will be able to find the answer a bit quicker now.