Cup-style mismatches like this are where I'm most careful, because the price usually already contains everything you know. 56% is only about 1.79 fair, so if the straight win is trading anywhere near 1.30-1.35 there's simply nothing left in it for me - I have a hard rule of skipping very short prices, since one lineup surprise wipes out a long run of them.
On your goal-line question: I'd rather express it as Over 2.5 or an Asian handicap than take a big -1.5 style line. The gap in expected points (1.93 vs 0.82) says Rennes should control the game, but control doesn't automatically mean a two-goal margin; sides that dominate lower-division opposition often win 1-0 or 2-1 after taking the lead early and dropping the tempo. So the question I'd ask is whether the margin distribution actually supports -1.5, not just whether Rennes are better.
The other thing is rotation. Managers treat these fixtures as minutes-management exercises, so I'd genuinely wait for the team sheet rather than model it in advance - if half the first choice front line sits, my 56% is not 56% any more.
If I played it at all it would be small, on the total rather than the winner. Nothing here is a sure thing. 18+, gamble responsibly.