Excited for this! I've made a bunch of Scottish maps of lordships, roads, earldoms, sheriffdoms, and ecclesiastical structure so let me know if you need any for reference or to be built for this TL specifically! Also, Scotland had great trade relations with German city-states, namely Hamburg, which William Wallace wrote to for aid during the 1st War of Indy. With a surviving (and even bigger) Berwick-based trade network, these links continue and Scotland could very well become a maritime power in the north sea.
EDIT: Just thinking about the butterflies, remember peace between Scotland and England is going to lead to a more stable northern England as well, and as a consequence, England might be able to better divert resources to Wales and France.
A journal article I read on trade said that Berwick spoke Flemish as a second language, so this can definitely continue!Correct. Scotland had very strong trading links with the low countries ('Fleming' is one of the most common surnames in Berwickshire even now) but also with Scandinavia and nations round the Baltic. In many cases the links dated back to the missionaries who went out from Scotland to convert northern Europe in the few centuries after St. Columba. Ships from Aberdeen (or at least the Aberdeen coast) traded fish as far as Köln (Cologne), through Flanders - cod was even called something like 'aberdaan' for a while, iirc.
But (and it's quite a big but) the majority of Scottish trade OTL before the end of the 13th century went through and/or was with England. So it's not just peace with England which will help the Scottish economy, but the maintenance of trade routes/links. It wasn't just the rape of Berwick, vastly reducing the North Sea trade, but the loss of routes to more southerly nations which caused the Scottish economy to cease growing / shrink (depending on exactly when you look at). Trade with France, Italy, Spain, etc took a long time to get going again, whereas all those countries had had regular trade with Scotland before the English wars.
EDIT: Just thinking about the butterflies, remember peace between Scotland and England is going to lead to a more stable northern England as well, and as a consequence, England might be able to better divert resources to Wales and France.