This will be a very pedantic reply. It seems like this is less to do with viable lineups and more to do with grind optimisation.
TL;DR If you can’t make the Centurion work in a 6.3 Lineup or the FV4202 in a 7.7 lineup, it’s a you problem. Running 5.3 tanks to support a Centurion is asking too much of the 5.3 tanks, likewise with the 6.7s - 7.0s
For Starters, the German 4.3 Lineup is downright diabolical, ignoring this Tank Destroyer lineup has me casting aspersions on your opinions (I say this in jest). Your talk of the ‘3.7-5.0’ jump has me concerned, considering that by your own commentary the VK 30.06 a 5.0 is an orphaned vehicle and by your standards the first viable lineup after 3.7 is 5.7, as the current Panther/Nashorn combo doesn’t meet the criteria you outline.
Now, standard advice I see is your line-up should consist of at least 2 “main vehicles” - not supports, not lower BR but equal replacements of each other in potential.
Given this advice being impossible to keep with orphaned vehicles, what’s a good rule of thumb to determine whether they can stand on their own (they’re the highest BR in your line-up), that they make a decent support vehicle for higher BR line-ups or even that they might end up as your main combat vehicle despite permanent uptiers (as was my discovery with the centurion. It performs better than charioteer in my experience).
For starters the advice regarding line-up compositions is the ‘rule of thumb’. What you have described is a system of contingencies not a strategy, the idea of back-up vehicles assumes that you are attempting to remain active in each battle for as long as possible as opposed to losing your primary vehicle and joining a new battle.
You’ve offered up Brittain as an example so I’ll start there. If all you’re concerned with is efficient lineups than you can easily skip the Centurion Mk.1 at 6.0 and the FV4202 at 7.3. While you’re at it you might as well Skip the 6.3 Lineup (Charioteer, FV4005, M109A1) as it’s basically a meme lineup. You might as well Skip the 6.7 Lineup (Ratel 90/20, G6 and Tortoise) also as it consists of very specialised vehicles with little to no synergy and shoot straight for 7.7.
On the other hand, if you’re a confident tanker and don’t mind playing conservatively, the Centurion and FV4202 are some of the most complete and effective vehicles in the game. The FV4202 suffers from it’s low top speed but there’s no reason why a competant player can’t take those into battle as a single vehicle and have enough success in battle to justify playing.
The only genuine advantage a lineup offers, is versatility due to the unknowable map rotation. Having both the Fox and the Centurion Mk.3 in a lineup is great, knowing you might play have to Fulda or Advance to the Rhine. At the same time the Cent Mk.1 and FV4202 are both reasonable versatible medium tanks.
Finally, the idea of viable lineups and orphaned vehicles starts to unravel completely when you consider elements such at enemy vehicle composition and the relative capabilities of vehicle. Lets pretend that the KV-85 is an orphaned vehicle (almost), Techinically speaking, there’s not a lot that the IS-1 and the T-34-85 (D-5T) can do that the KV-85 can’t do almost as well. The firepower and mobility aren’t enough of a different to make a difference.
Another inspiration of this post is trying to decide if the FV4202 is worth adding to Cent MK3/Caern/Conq/Eland/ or even replace one of them.
The FV4202 is basically a Centurion Mk.3, but slower this is a very reductive description, but it’s true enough in the context of a video game. Lets say you have a lineup with 4 x 7.7 vehicles and the FV4202 is the last vehicle you spawn. The match is basically over at this point, you’re either getting kerb stomped by the enemy team and the FV4202 is being spawned to shoot 1 or 2 spawn campers before dying, OR, you’ve basically won and you’re spawning the FV4202 to cap a point or mop up the least 3-5 players left active on the enemy team.
Also, if you’ve already thrown away 3 vehicles in a battle, who even cares about a 0.4 BR difference at this point.