As it's pointed out by the articles, it's essentially a matter to deal with Saharian peoples, rather than a full fledged conquest.
Simply said, Lake Chad was relatively far, and you had to cross not that hospitable terrain, and Maternus did with the help of
Garamantes, a powerful people of the actual southern Libya, the whole point being crushing rebel clients of the king.
We're talking then not of an offensive, but about provincial politics, which involved not that known people (no other mention of Flaccus elsewhere) and probably much auxiliary support.
Not that Chad Lake's surrounding were uninteresting : Kanem-Bornu and its possible ancestors (Agisymba?) flourished there. And should Garamantes collapse earlier, and/or Romans have a real African policy,
maybe Chadians could enter in constant relations with Romans, more or less the same way Garamantes or Nubians did.
The greatest African expedition happened durinh the Severian dynasty, from Leptis and with a greater knowledge of the region, after all.
Basically, such expedition would probably use the old trade roads (remember we're talking of an era where camel is virtually absent, so where such roads are more harsh and take longer to cross) : Fezzan/Gao, Oranais/Niger, Tripoli/Tibesti.
But one should remember that expeditions in Africa existed before : Hanno's periple or Pseudo-Scylax, for instance. But they were rare, and not that motivated. These trade roads didn't let much traces, and the exchanges may have been quite poor (and certainly virtually inexistent in comparison with the medieval trans-saharian trade).
What should motivate Romans would be military purposes, not economic, and would likely limit ambitions in the region. And Berber Kingdoms as Garamantes aren't going to make it easy.
Basically, for having something more, you'd really need to have a far more important trans-saharian trade, that may push Romans to control its northern trade cities/region. We're, IOTL, really far from that, and I don't know how to realize it giving the motivation have to come from Romans (trans-saharian or sub-saharian entities not being unified and strong enough to have the incitative).
Maybe (but I don't know nearly enough to say if it's possible : my guts says it's not easy) a flourishing Nok civilisation that manages to form not only a strong entity of its own, but influence its neighbours to undergo similar evolution?
I imagine that the expeditions were pulled back or abandoned at the first crisis, if they were any large,
Not exactly, you had more or less regular contacts, and you had other expeditions that followed. Not that they were costly (not particularly large or "threatening" politically) but it was clearly not a major or even secondary focus.
