I have just returned from a 14 day train based trip to Europe. I traveled with the 2 wheel Maxlite 22in carryon and a 24L backpack. I did not check bags on this trip but have also used the 22in Maxlite as a checked bag (doesn’t meet European airline carryon standards) with the backpack as carryon before.
You need to be prepared for exactly how steep and narrow the steps are getting on the train, don't underestimate this. As a shorter person I find it doable but challenging even with a 22” carryon. (This is where WengenK’s comment on being able to pull the Dakine up the steps makes a lot of sense.) I witnessed many petite and older people REALLY struggling to get their roller bags up the stairs. I would want both hands to lift mine up the train steps (I go up the train stairs and turnaround as my husband is lifting it up to me so I don’t have to do this).
Additionally, you will be using stairs in many rail stations to access the platforms. Some will have escalators but we found 50% of these not operating this summer, ironically mostly the up escalators. You may have access to an elevator, but you don’t want to count on it, especially if you have a quick transfer to another platform. Although at the beginning of the trip I carried my 22” roller up and down with one hand no big deal, later in the trip I was feeling worn out and ended up using both hands to hold the bag in front going up and down stairs.
The point of this long description? IMHO you really need to think backpack not weekender. The weekender is great when rolling your luggage, but it has to be removed when going up and down stairs and getting on public transport. You would have both hands full and I worry that can also be a security risk. Train/metro stations are places where you want to keep your hands on your luggage at all times. You will invariably end up setting one down to manhandle the other onto the train. Yes, backpacks aren’t fool proof since you can’t see what is going on behind, so just don’t put valuables in there and get a small lock to lock the zippers closed, but they are hands free! For trams and buses you may need to push a button to enter or exit. All the ones we used were tap your CC or phone to pay. IMHO, you will want one hand free at all times!
Next, the 42L Cotapaxi . It sounds like you are choosing this just to have room to pack more for carryon in case your bag goes missing. It is 4.2lbs- OUCH! that seems heavy for a backpack that you are carrying all over Europe for 81 days. you may regret it. My pack has no interior organizers except a padded laptop sleeve and only weighs 1lb, 5oz, 24L volume. At times I have used it as the only carryon I can fit a raincoat, sweater or jacket, 2pr underwear, 1pr socks, one extra top maybe two if they are small, tech bag (charging cables, power adapter, battery pack) a couple snacks, Rx and OTC medications, essential toiletries, a few papers with essential itinerary info, and eReader. In a pinch this would get me through 2 days before I’d decide maybe I need to buy a couple of things. Pants can easily be worn 3 days as long as you don’t spill your coffee. Unless you have an extremely short connection, I believe the odds of your bag not arriving (much less going missing for many days) are so much lower than the regret of a heavy pack. On these forums everyone tells you the problems and what they do and it all sounds great, but not everything is doable in every circumstance. I think you need to prioritize! If you were traveling and attending a wedding or work conference immediately after arrival or catching a cruise where luggage can have trouble catching up then maybe the large Cotopaxi bag would make sense. I just don’t think it would be the end of the world for your trip, and I am very risk averse. Don't underestimate how exhausting this kind of travel can be, look hard at every single ounce you are packing.