I have a sort of poison eye for le Sportsac bags even though I see their practical appeal -- but I have a sugar eye for plaid. So I come out about even on this bag.
I do think a muted plaid is a superb idea for you and I like this one with the items you have chosen.
I adore a print shoe (so much more than I expected) and also print belts. I don't have a print bag but I would strongly consider one. I do have two colour-blocked bags (actually neutral, since they are bone and black, and black and white) and they are absolutely among my favourites -- so much more versatile than I would have guessed. If you wear a lot of solids then this plaid should be perfect. I say try it.
Also, last year Zara had an inexpensive plaid wool backpack. I don't know if they will make something similar this year.