You asked about the surfaces of canal paths NE of Tiverton and E/NE of Taunton.
I think you might wish to ask for advice from others on the Camel Trail north of Bodmin.
I have ridden it on 25s (uphill (50m in 10km!) both with and without a small load) but I'm conscious that my surface tolerance is quite high. Your initial Day 1 route included that section of the Camel Trail which is why I included it in my suggested Day 1 for you. The top (NW) section of the Camel Trail coming down from Padstow, through Wadebridge to the trail junction just N of Bodmin (where your route joins it) is a lovely fast, flat surface - the first 11km alongside the Camel estuary. The trail north from Bodmin is a wonderful ride of about 10km but a bit rougher.
Given that no sensible rider takes the dual carriageway A30 directly across Bodmin Moor, one has to go north or south of the moor. The route to the south is a lot hillier - Minions is a highlight (but a bit shorter if going to Okehampton). It does line you up for crossing Dartmoor (ie via Tavistock), if that's desired. Otherwise going to the north (ie through Davidstow), there are (essentially) 3 ways across the River Camel: Padstow -Rock foot (and bike) ferry; at Wadebridge (which can be accessed via either the A39 or via Padstow and the aforementioned Camel Trail (but that's a fair diversion)); or through/past Bodmin.
Having chosen Bodmin there are three ways to get north of the moor: A389 then B3266 to Camelford, Camel Trail (the route you drafted); or on minor roads // A30, then past Blisland and through St Breward. Your choice.
My Day 1 choice was to do the A3075 to Newquay, head NE to Padstow, and take the ferry which sets you up for the B3314 from Rock all the way, almost, to Davistow. Adapting that to get to Okehampton gives you this:
LE-Okehampton 174km + 2166m.
HTH