Hello :-), sorry i`m late…
SUW to Lauder is all ok and mostly rideable with a cross bike, there is cliff top trails from St Abbs west to Dowlaw (signed coastal trail) but a wee bit pushing and above cliffs so is a bit vertigo ish 🙂
Some nice green lane type trails around Coldingham marked as white roads mostly on OS maps and roadside wooden finger signs,
Berwickshire has a lot of these trails, esp around Chirnside, Ayton, and Duns, Around Duns Castle estate is some interesting trails, rooty in places but it is a very muddy clay soil there, there always rideable but you will get minging in winter unless its frozen,
The Herring road south from Dunbar at Halls Farm has mostly been decimated because of the windfarm up on Lothian Edge 😥 but it can be ridden on the re routed new paths, not great for a mile or so but will get you across to the Whiteadder Resovoir, shooting tracks and the pylon maintenance track running west of Mayshiel are all good, the bleak rolling hills of patchwork heather grouse moor are usually deserted and is what makes up there great,
chugg08s route would be ideal too,
CX bikes, 29er ridged or a good old hardtail xc bike is perfect for around all these places, 🙂