Oh hay, me too.
So I got lots of potential directions for you if you'd like to hear them.
Would you be interested in trying 1man officer squads that can be merged into riflemen? There's a lot of ways that sort of thing could go, so I'm interested in how radical you're trying to get with this.
Anyway, I think the biggest struggle with WFA teching is that it's a challenge enough just to order their unit rosters in a way that makes sense, let alone balance the pacing of those units to the other factions in the game.
For reference, the M8 Greyhound worked in vCoH because it could, potentially, beat anything that Wehrmacht fielded from it's T3. (A Greyhound could circle strafe a stug to death. This was unlikely and all, but it was still a threat.) It was great for rushing a nebelwerfer. Though the puma could be upgunned to annihilate the M8, that was an axis investment that reduced the puma's AI. The M8 wasn't an efficient counter to T3, but it could punish rushing T3 effectively enough and that is what mattered.
USF in CoH2, however, has nothing similar to handle Ostheer T3 since it has the P4 and Ostwind. The Stuart can handle Luchs, but isn't really great against Pumas. The AAHT and M20 then struggle for a role for their costs. I feel that this is the reason that USF has had to rely on M10 callins so long: They have to have something that actually competes. Adjustments to USF teching needs to address the pacing of both OKW and Ostheer.
M10s and Shermans I think do this well, since both Axis factions have a lot of resources to handle cheap mediums. (Thanks to Soviet design and T34/76s.) Shermans and M10s are both solid units that are able to trade efficiently against Axis mediums, but are outclassed by panthers, tigers, etc. Not only that, the Puma can harass both (more the Sherman) rather effectively. But the point is the M10 Wolverine and the M4 Sherman are balanced counters to both Axis mediums by being roughly equivalent but cheaper, but ultimately outclassed by the more expensive big cats.
The Jackson, past and present, does not share this kind of balance with Axis mediums: it's too expensive and performs too well. In many ways the Jackson may function well as a doctrinal unit, switched with the M10. Though as an idea, it could be interesting to put the M36 Jackson behind a Brit-style side tech upgrade. Choose between Jacksons and the Scott. (Or something) But IMO the M10 really should be migrated over to being a stock unit for USF. I highly recommend it when trying to come up with a solution for USF teching.
WFA have had chronic issues with their indirect fire options and countering garrisons. Allies (Well all the factions really) have had chronic issues with having many doctrinal components that should really have been stock. (The 2CP community commander is pretty much a testament. That's how CoH2s core incompleteness was addressed: as commanders.)
As far as the pack howie is concerned, I think it should just be switched outright for the mortar halftrack. Pack howie could be an airdropped support weapon option for any of the commanders with airdrops. (OKW could ditch the leig for a mortar halftrack too, for better gameplay AND continuity with USF!
) The USF mortar team should probably go away though. I mean, as it stands, USF has the mortar team, pack howie, and the Scott as stock indirect fire options and has the mortar ht, calliope, priest, and an increasing number of units that can callin offmap artillery in commanders.
I really think, at the very least, the USF AAHT be swapped with the doctrinal halftracks. Especially now that FRPs have been nerfed I don't think it's the craziest thing in the world for WFA to have reinforcing halftracks.
TL;DR:
I think the linear tech idea isn't a bad one, but you should consider making the M10 stock.
As far as Brits are concerned...
I really like Planet Smasher's Artillery pit concept and function, but I really like the model used for your version Aegion. The single team weapon nature is best for gameplay I believe. Quite frankly an equivalent of a USF fighting position that a vickers or a mobile mortar squad, (or an 6lber or land mattress, etc) could use to gain hulldown effects I think could work nicely. The double mortar nature of the existing mortar pit is a little much IMO.