Frankly, I appreciate the thought you put into your assessments.
Re: 3rds. The use of the word “impossibility” with respect to 3rds, is, IMHO, too strong. Consider a simple C major arpeggio, low to high. With a bass in 4ths, you might use finger #2 at C (string III), #1 E @ II, pivot up to #4 G @ II, #4 C @ I. With a bass tuned in 5ths, I try to set up the tonic with my 4th finger. The pattern would be: #4 C @ III, pivot back #1 E @ II, pivot up #4 G @ II,(same as 4th tuning) #1 C @ I. From a logical perspective, there is a greater margin for error using 2 pivots instead of 1. However, daily warm-ups with thirds are a must. Minor 3rds are a significant challenge, depending on the layout. Consider C minor. I start with 1st finger C III, pivot up to #4 Eb @ III (similar to 4th tuning), pivot back #4 G @ II, #1 C @I(as before). My approach to the technical aspects of playing bass is about learning the patterns, not what is easier. You are going to shift and pivot to play the Bass regardless of tuning. But to focus on the disadvantages of playing in 5ths is more about preference and attitude than “different strokes.”
Re: 7ths are a colossal PITA. It requires a whole shift back for the Dominant 7th. Vomit exercises with 5th tuning are a must.
Re: Simandl. (This is borderline heresy.) Why would you want to apply methods designed for 4th tuning to performance on a Bass tuned in 5ths? Consider the terminology of “first position.” Bass players must cover significant real estate. If you consider 1st position on a Cello, it would be “1st neighborhood” on a Bass. Simandl is traditional and stands the test of time. But in 5ths, I consider Joel Quarrington’s publications. Also, Dennis Masuzzo has a wonderful publication with a few excerpts with fingerings for bass tuned in fifths. If you are looking to get used to the rumble of the low C to Eb (IV), Isaia Billè published a method for 5-string Bass in 4ths (low B/C) c. 1922. Pick and choose exercises at your discretion as it is in IMSLP.
Re: Jazz. I am limited in experience, but I do not see an advantage playing in 5ths for Jazz licks, with the possible exception of creativity. If you are reading a chart with a cord progression of: C, F, G7, C, it doesn’t matter how you tune, there are certain things you must do. Although, Red Mitchell cooked with it.
With all due respect, I don’t ask for agreement, I ask for consideration.