Have used a Surly Porteurhouse bag on a couple of short, non-technical bikepacking trips now. Fit onto a cetma 5rail nicely. Solid bag.
My nearly finished build has a Surly 24pack rack, and I intend on switching between using a wald 139 + net, and the porteurhouse bag depending on the trip (the bag isn’t one that’s designed to fit into a basket like the above)
I reckon you’re overthinking this. The Tera Rosa bag looks perfect. Well thought out design, not too expensive compared to others, plus local supplier who you know will give good support.