In New Horizons, trading items is needed to complete your catalogue since you only get one color variant in your store (I think you can get other colors from balloons and villagers but that's a really tedious way to catalog everything) and I would say it is definitely encouraged to trade for items you don't have. It's a big part of the game and I think it's not making things too easy, it's actually pretty difficult to find a lot of trading partners to get all your wishlist items! Makes for another challenge in the game.
That being said, I wouldn't want to catalog all orderable items from one of those island that offer that service. When it comes to that, I do feel it makes the game less fun and too easy in a way. Obviously I don't judge anyone for doing it but for me I just think it would make me less excited to look in the shops or spend my bells because I already have everything catalogued so what is the point?
Still, in New Leaf I did use hacks to decorate some of my town and houses, which of course made the game easier, but I didn't think it affected how fun I thought the game was since I was focused on decorating and not on playing the other parts of the game. I had already done the fishing, bug catching, furniture hunting, extensive trading, et cetera in my earlier towns and so at that point I didn't want to play those parts of the game anymore since I didn't find them as exciting and fun doing them over again. I think the same could happen in New Horizons if I ever were to get a second Switch (I'm not deleting my town, I would cry) I might buy one of those huge cataloging services just so I could focus on other things. I don't know though, that could definitely change...
I would say that filling the museum with stuff from other players takes the sport out of it in a way since you can get all of them on your own island eventually (glares at Redd), but like others have said, there are just so many furniture color variants. It's grueling to rely on tree shakes and balloons to supply the variants you get that aren't "native" to your island.
It does make collecting items too easy, but I see no issue with it and enjoy 'picking up the pieces' I'm missing to complete my collection(s). Certainly a lot faster too!
You're able to skip grinding an event for hours, or time-traveling continuously to check the shops to get the items you want. Imagine the time spent on loading screens alone! You can also avoid feeling your heart sink as you enter the shop to see an item for sale that was there just a few days ago. Plus, you don't have to wait a year for the event/seasonal items to come around again if you didn't get everything.
I recently cataloged all 'orderable' items because I was in a decorating phase and had a limited selection. It took two hours to catalog everything, and it made decorating more enjoyable. Good trade-off.
This kind of thing is going to depend on each person's view of the matter. I personally would find the game terribly boring if I immediately grabbed all of my favorite items from a giveaway or another person. It's a wonderfully exciting feeling that brings a lot of joy when i find an item on my own that i was really hoping to find.
I get wanting to complete the catalog on your own and maybe having that sense of accomplishment but I'm definitely not opposed to getting help from other people. given the number of variants there are for some pieces of furniture I think it'd take x10 longer if you were looking to obtain them all by yourself.
Not really. I think I have seen every single item that has appeared in my Nook's Cranny now at this point and going in and out of Ables is so tiring. Like I have spent a lot of of hours trading at the time. I wouldn't be surprise if it's 100 hours just trying to trade of my actual game play and I'm not even done with my item or clothing catalog.
Even if I finish the catalog, there's still the rest of my island and house to decorate. And the finding and befriending villagers as well as the critterpedia completion.
They made it more difficult with making only certain color variations available in your store (and not being able to just customise to the other colors). I thought that was dumb so I catalogued everything. I do miss getting excited about seeing that one piece of furniture finally being in Nook's, though.
I have no issues with trading but I have no desire to catalog everything so I just stick to small trades when there's a specific thing I really need. I think if they had done the item variations a bit differently then playing fully offline would be more feasible but with how it is now it's almost impossible. I'm not going to decorate my room with a red imperial bed, a blue imperial table, and a black imperial chest. If the colour furniture you got at Nooks was the same for an entire furniture collection then that would make a world of difference.
As for the museum I don't buy museum items from other people just as my own personal rule. I might end up time travelling to complete my art section which I guess is cheating in its own way but I would rather put in that TTing effort myself than just buying the art.
I wanted to start off collecting everything myself this time and not make this game another New Leaf experience, but I kept finding the same color of furniture in the Nookling's shop, I don't like playing at night time in game, so I'd hardly find Wisp, Redd doesn't visit often, and shaking trees or shooting down balloons doesn't guarantee you'll find any new furniture. All of this made me give in and I don't regret it. I don't have to deal with that nightmare again, and I can still enjoy the game for other aspects, like collecting seasonal DIY, decorating, fishing, bug catching and talking to villagers.
I also went to a gift Island where almost anything could've been picked up and taken home. It felt like a dream. I enjoyed the experience very much, but there are somethings I also didn't pick up for the sake of trying to stay invested. Eventually I got bored, but it was mostly because the villager's dialogue was boring and I guess everything just felt repetitive.
You can also avoid feeling your heart sink as you enter the shop to see an item for sale that was there just a few days ago. Plus, you don't have to wait a year for the event/seasonal items to come around again if you didn't get everything. 
