Talgarth-Llangors-Bwlch-Llangynidr-Gilwern is another option to minimise distance on A roads
That is one clever option... basically you get rid of a climb at the cost of only 4 extra km... 
In what way would one "get rid of a climb"? The figures I offered above suggest 7m of climb saved.
Talgarth to
ChainbridgeRoutesheet 2017/2018: 41.2k + 325m on A379 and A40 and through Abergavenny
Probable routesheet 2019: 41.3k + 413m on A379 and A40, then right at Crickhowell to A4077 to Gilwern, then B4246/B4269 through Llanfoist to Llanellen, and south on A4042 through Llanover and left to Chainbridge
@Somnolent's option to minimise A roads: 44.5k + 433m going via Llangorse and Bwlch to Llangynidr, then B4558 and staying on A4077 through Gilwern, then B4246/B4269 through Llanfoist to Llanellen, and south on A4042 through Llanover and left to Chainbridge