My opinion as to this being an engine performance issue is just based on the symptoms and a gut feeling. Nothing more. Sometimes the most insignificant of things can cause headaches. Tiny vacuum leak, corrosion on a spark plug boot, aged spark plugs, etc, etc.
I’ve experienced converter shudder on my Fords a number of times after the fluid ages so it gets changed every 35k miles. Over 40k miles and the subtle signs begin.
RPMs never drop and there is never a shudder on acceleration. There can be a rumble strip sensation around 45 MPH but the tiniest application of throttle makes it go away.
Again, my opinion is based on symptoms and a gut feeling along with the fact that your car’s problem seemed to come on suddenly so I tend to think engine. That does NOT mean the engine has serious problems.