Being a 'roadie' with 32mm slick-ish (Gatorskin Hardshell) tyres on my Cube Cross Pro (a gravel bike), I adjusted probably a third of my route away from the official NCN #8 and onto quiet A or B roads. This brought down both the length an the overall elevation and meant I could complete within three days. Day 1 Holyhead to Llanberis, Day 2 Off Bike - Hiked up Snowdon, Day 3 Llanberis to Builth Road, Day 4 Builth Road to Cardiff.
I wanted to keep Day 4 short as I had a specific train back to London booked, as such Day 3 was nearly 200km.
Great lanes and good train line conversions - a nice route. Only one puncture - on the Taff Trail - don't think it's ever been swept(?) - and that last bit from Merthyr Tydfil into Cardiff was probably the hardest bit to navigate. Brecons are ok but there are a lot of trees; if I go back to this area I'd probably go to the Snowdonia or Cambrian bits - the views are just more expansive. Anglesey is lovely (and quite too) but also pretty flat and samey.
Overall really nice experience, friendly people and cars were by and large respectful. Handy that there are good train stations at both ends of this.
btw; looked at the David Brailsford 50/75 routes too (in Snowdonia) but also watched the streams of traffic on those roads around the Snowdon area and was glad I had avoided. Probably fine in a bunch of 3-6 but less fun solo.