I agree with @boombang. If sale on the Tarmac is final and the bike just plain doesn’t fit then you will be stuck with it. However, even though it is also buying without seeing, Canyon does have a return policy so you can at least get a feel for it before really committing.
Also, your concern about bringing a bike that you bought online into your LBS for service makes no sense since you will be doing that either way. They’ll see no profit from either purchase whether they sell that brand or not. Buying used probably ‘hurts’ bike shops in just the same way that buying a direct to consumer brand does.