Nice idea Nick, Jack,
I'm currently doing a HF rig based on works from the book by EI9GQ. [ Not a qrp rig. No real attampt to lower Rx current)]
I first built all the sections as DIY boards then consolidated them into the magic 100x100mm for Chinese supplier.
Receiver is working, low level Tx side all tested up to 1W level. Uses 9MHz crystal filters available on ebay at least for now, from a "Telrad" board. Might possibly build own filter also as the Telrads are bit wide. No CW.
Band pass filters plug into a 100x100 motherboard and are used in Rx and Tx modes.
I'm not suggesting we go down this road but that the idea of using 100x100mm boards has a lot of merit.
I'm yet to decide on the control Micro but for initial testing uses a NANO and Si5351. Your suggestion Jack to use a more capable Micro is a good one.
BPF, one fitted to mother board.