Did not the Romans have more advanced techniques for farming?
For mediterrean enviroments, sure.
The Empire, after all, supported a population much more dense than Northern Europe.
Because it was climatologcially-naturally more suitable, not becaus eof technologcial improvments.
Look. I like AH because it lets you explore possibilities that did not actually happen.
Yes, but pondering it on the net involves discussion, including the plausibility question. Its a valid part of AH discussion. But for the sake of said discussion, lets assume the Romans do manage to gainc ontrol o germania. maybe tehyr emotivated by trying to get all those unrestful peoples under their control instead of having constantly to worry about the Rhine border.
Its... possible. However, Roman culture and society was largely urban. Now, fortunately for them and admittedly, the Romans were also very good at founding cities, which they would have done in Germania (there was a report in TV some time ago - apparently the planned capital of transrhenanian Germania would have been quite near to where I live, actually.) However, outside their cities and forts and maybe some streets with way stations, out there in the nearly impenetrable thick forrests, I assume Roman rule would be shaky and superficial at best. Of course, the Romans let indigenous political and legal systems survive everywhere, but they made sure to always have the last word, so to say. This would not be guaranteed in such an enviroment. So to say, I think any province Germania Magna on a map would not have to be colorued Roman, but striped.
Now, long term development might well change that, its true. But Valdemars list of abandoned provinces of OTL speaks for itself. For the Romans not to abandon Germania would require them having no problems elsewhere (difficult) or having some extraordinary success in Germania due to some extraordinary circumstances (which always can happen, I suppose, but not regularily so).
In a less warlike atmosphere, perhaps after a century of secure and centralized Roman rule in Germania after Augustus' time, with growing townships, land clearances, drained marshes, and the introduction of the Latifundia, would the farming tech be much of an issue, or would it even remain at the same level?
Well, it did develop in the Middle Ages IOTL, so its possible. Still, I dont think you can pin it down on a warlike enviroment, its more the natural enviroment that was the problem...