Just use Komoot it’s the best route planner as you can tell it what sort of bike you are riding and it plans routes accordingly.
Obviously it’s not 100% but it’s a great start and you soon realise which bits to reroute once you have actually risen them.
It also classes roads as highways, B roads and county lanes and you can highlight each category and reroute to avoid busy roads.
It also classes off-road sections into single track, fire roads, bridle ways and footpaths etc and you can highlight each category and reroute those.
It also has suggestions from other riders that you can incorporate into your route where they leave descriptions and pictures.
Some people don’t like it but I can’t see any downsides with it.