In the strategic picture, this is little different from Europe as a whole centuries later: with Magyar, Viking and Muslim "incursions" occurring or threatening for generations. Romanized Britain is just "Europe" in miniature. Any raid can occur from any of the three enemy groups at any time, from land or sea.
With this exception: Europe of the 8th through 11th centuries, as a whole, was much more powerful than the three invading groups: Romanized Britain was weaker and doomed to being conquered.
Britain rapidly breaks up, all Roman centralization lost in the anarchy, with mostly weak prey waiting to be set upon. Lots of fun for the invaders! The "Romans" can only hope to stave off the inevitable for a few years with a tight defense of limited areas.
The most powerful element are the "Germans". The local invaders (Picts and Celts) have to watch out for the Germans too, because they will beat on anybody they find.
Alliances would be temporary in the extreme and fraught with treachery. More fun!
I would handle this with a multiplayer campaign. Ideally that would work best. A solo campaign would generate an invasion or invasions. The British Romans would be waiting for the next blow to fall and respond to it with what they have. I'd give them a good capability of mustering quickly: they'd certainly be used to doing that! But limited manpower. If two or more threats occur, they can only reasonably respond in the field to one at a time: or risk being outnumbered.
As for keeping a strict log of passing time, I'd dispense with that altogether and just generate successive invasions. A "recovery" roll for sacked/pillaged towns and districts between invasions would reflect the passage of time. Make this a slight chance for little time between invasions, and increase the chances of a partial or full recovery if an area on the map hasn't been hit for a while: such an area would attract refugees and recover more swiftly due to the restoration of population.
There are limitless ways to generate strategic actions by the invaders if they are the NPC entities. Ditto, if "you" are an invader and the Romans are the NPC defenders responding to you.
I've done quite similar things as your campaign suggests in my "Raider" games: but not for lengthening years by now………. TMP link
