The main difference between the Slav and QGD is that black in the Slav plays c6 and delays playing e6, while in the mainline QGD, black plays e6 first and might play c6 or c5 later depending on the variation. The initial pawn structure is the big difference between the two and can result in different strategies. For example, since the Slav delays playing e6, there are many variations in which Black's c8 bishop goes to a more active square like f5, whereas in the mainline QGD, that possibility is almost always sealed up after black plays 2...e6. Additionally, an idea in the Chebenenko (and sometimes in the Slav) is to threaten to capture dxc4 eventually and gain the pawn or some tempo with ...b5. If you're playing a mainline QGD as black, you're almost never going to do that. Slav and non-mainline QGD's in general are usually more aggressive than the mainline, although the Slav is also well-known for its high draw rate in some variations. The mainline QGD is generally regarded as a solid opening, but black has to defend for a period of time before he can consider counterplay. Finally, if you dislike playing against a Catalan setup as black, many Slav variations have the advantage of equalizing quickly against the Catalan, while mainline QGD structures for black are, again, somewhat passive versus a Catalan setup.
On the other hand, the QGD mainline is still playable and used quite frequently. It's considered to be a slightly passive but pretty solid opening in general. It's also a very straightforward opening to learn, and with regard to you saying that you don't like highly tactical openings/middlegames, there's not much tactical motifs you need to know in the mainline as far as I remember. I played the QGD mainline as black for a while, and many positions in the mainline are similar to each other so the ideas carry over quite well.
As someone who plays 1. d4 d5 queen pawn openings as both white and black, I would rather play a Slav-like setup than a QGD mainline as black, and there are times when I do not enjoy playing against it as white. If you don't like some aspects of the Chebenenko or the Slav in general or the mainline QGD, you could also consider learning the Janowski (1. d4 d5 2. c4 e6 3. Nc3 a6) which is what I play now and combines some features of both. At the end of the day, both are playable and it's up to personal preference.