I imagine a 5 man squad, similar to the Assault Grenadiers, but armed with StG44s instead.
And yes I'd also think having the option to upgrade with Armor and Single/Double flamethowers would be nice but that'd be dependent on what would be most balanced and so forth.
Maybe they could have a 3rd utility upgrade of some sort that allows them to for example repair vehicles and plant demolition satchels like the British Sappers to remove cover and obstacles? Or your smoke and charges idea.
Could work.
What i said are not my ideas, it's how the unit is. Seeing that you want the "whole package", let me explain you why it wouldn't work.
If a 2x flamethrower unit is gonna exist it's gonna have to be RET levels of weak with absolutely no extra utility mixed in it. Maybe a satchel charge but the unit would had to be Pio levels of RA and Mp40 Pio levels as opposed to PG STG.
If you want a single flamethrower, it could have slightly stronger models (PG ones with STG) but again, no utility whatsoever and 4 models. Any other combination of utility, flamer and BALANCED requires them to be Pioneers.
The 1x/2x flamer is a dead end however you look at it. Same with implementing an armor upgrade if you want a unit as strong as vanilla model PG.
At the end of the day i think you are forgetting about either Breakthrough equipment and Support package.
Pios can get access to satchel charge, 5 man squad or destroy cover.
PG can get access to repair, improved performance near vehicles, mark infantry or smoke grenades.
The only "unique" thing available to be seen is probable a 5 man PG squad with destroy cover, smoke and probable an incendiary grenade. Probable limited to a single squad. Maybe a BP3 upgrade which replaces the incendiary grenade for a satchel and provides 0.25 armor (similar to RoE Anvil upgrade) for scaling and keeping the theme.