Confusing the two.
Substituted 6 chords are those triads or 7th chords where the 5th has been replaced (substituted with) the 6 - I think most cases, if not all, are M6 above the root.
Note, this is not Jazz, where such a chord would be seen as a 6 chord or 13 chord with an omitted 5th.
5 - b6 - 5, spelled that way, would likely be a chromatic inflection of whatever triad it was on (let's assume I for now). Would probably have no reason to ever call that I - I+ - I as it would be unlikely to be of such a duration that the + chord was a "chord proper".
5-#5-6 could be found in two states. One where the #5 could simply be a chromatic inflection and NCT, thus I - I - IV or I - I - vi, etc. Again, duration would be a primary factor here (besides context of course). And if something like that ever did appear with a longer duration, most people would label it I - I+ - IV despite it's more obvious contrapuntal origin.
The other state would be where the + chord is acting as a secondary dominant to IV (or vi), thus I - V+/IV - IV.
Again, "instability" has nothing to do with function.
As far as the Blues I7 is concerned, jazz and blues are different styles than what this type of analysis was intended for and while some of it is a 1:1 correlation, much of it is trying to fit a square peg in a round hole.
We really shouldn't even be using the word "Tonic" to describe the I7 "tonic" in blues. What people are trying to do is say "it's acting in a similar way to the way the tonic acts in tonal music" but it's not really the same thing. After all, we call the "tonic" in Modal music the "Final". Why? Because while it's somewhat similar on the surface to a "tonic" proper, the two are *conceptually different*. We could call it a "blues tonic", but the more accepted academic term is "Center". Unfortunately, "Center" is a little too broad for most people.
Here you go:
When "acting like the 'tonic'":
Single final note in a modal work: Final (D)
Tonic Triad in CPP music: Tonic (C-E-G)
"tonic" 6. 6/9, Maj 7 etc. chord in Jazz: Jazz Tonic (C-E-G-A, etc.)
"tonic" Dominant 7 chord in blues: Blues Tonic (C-E-G-Bb)
"Functionality" also needs to be re-defined in various styles. For example, in Jazz, many common tone progression are found that otherwise don't make much sense in terms of traditional root music - but are they any less functional?
Likewise, a bVI7 - V7 is a somewhat different principle in Jazz and Blues than a Ger+6 - V7 progression in CPP tonal music and to conflate the two is missing out, losing, or otherwise making unimportant the distinctions between them.
Many people already do they - they just say "it's in the key of X" when the music is not really in a "key" at all, and they say "it's the tonic" when that's not exactly the term they should be using (but since they know no others, and have heard that misused so many times, it no longer matters).
If we re-define "tonic" in Blues to mean a I7 chord for example, what does that do? For example, many blues or blues based progression actually wait to introduce I7 until the 4th measure such that it acts like a V7/IV. So let's say it is a I7 throughout. Is it:
I7 - I7 - I7 - I7 - IV7, or
I7 - I7 - I7 - V7/IV - IV7, or
V7/IV - V7/IV - V7/IV - V7/IV - IV7, or what if there's a quick change (IV in the 2nd bar). Does that change things?
Blues and Jazz have gone to a largely non-functional labelling system. In most cases, they are labelling the chords for playing, not for analysis. Yes, they often do try to keep secondary functions when obvious, but the labels are being used as "just chord names" in a lot of cases. They are not necessarily tied in to functions. You can see this very readily if you read anything that says "I, iii, and vi all have tonic function". Not in CPP music they don't! But in Jazz, they're "treated as tonics". They're "tonic substitutes" or whatever we want to call them (iii being a "rootless tonic" you know).
The labeling system for CPP tonal music doesn't work for concepts it wasn't designed to handle. So new concepts need to be devised. IMHO it's OK to build on the work that's already be done, but stylistic differences should be pointed out - don't call a TTS bII7 - call it "V7ts" or something. Don't call a bVI7 a German + 6 because that's not what it is. Wholly different concept (albeit similar sound). Call it bVI7. We don't call a C6 an Am7/C unless the context is clear. And we don't have "Iadd6" in CPP music, so come up with another label for it...
otherwise the distinctions are lost.
Steve