I haven't had a slightest idea whether to continue working on this project, so I've uploaded the new design files, and will be uploading the Gerber shortly after a week or two once every tingling gut-feel to fix board bugs checks out.

At this stage, the board has been shrunk to 24x24 mm which can be panelized easily in a 50x50mm PCB for cheaper manufacturing. It should be simple enough to change the balun filter IC for another according to Texas Instruments application note SWRA250A for operation in 433MHz, 868 MHz or 915 MHz individually.