This technique is the way my chef taught us in a long ago class and old school bakers used it with non-crusting buttercream frostings. She said it was the way she learned to make cakes in Germany.
We used cake rings, that were 4 inches tall. Our cakes were torted into thin layers and we used an Ateco #3 tip to fill. The rings are just wider than the cake itself, because the cakes shrink a bit in baking. You piped a thick ring (like you are piping a dam) at the outer edge right up against the ring, making sure to build it up. Then you used the tip to make the buttercream filling layer, and then build up the edge again, and put in the next cake layer, continuing until you were at the top. Then you covered the top with icing, and smoothed it over the top, using the ring as a guide. It chilled overnight in the ring. Then we used a torch to heat the outside of the ring, lifted it up (or down, depending on what kind of board we built the cake on) and got a perfectly smooth finish. To me, it was a lot of hassle, and you’d have to have rings in every size and multiples of them if you were doing production work.
She also taught us to use the large icing tip, saying it would deposit a uniform thickness on the sides, and she’s right. It works great, and is fast. You aim for the top, spin the cake while you pipe out this big ribbon of icing, then use the spatula or bench scraper to take care of the excess. The problem for me is it takes up another icing bag and I have to stop and fill it frequently when I’m busy. Plus the extra large bag is hard for me to handle.
The bench scraper method works because it is sturdy enough not to bend - many beginners have a tendancy to hold an offset spatula at an angle, and not straight. A woman I know who worked for Mike McCarey (Mike’s Amazing Cakes) said they used the long (12”) paint maskers there instead of bench scrapers simply because they were longer and could work on their tall tiers.
You want to store your tools carefully, because if you are not careful, your offset spats can become bent out of shape and then they are no help at all.
The key is to practice and know what works for the kind of icing you are using. Some techniques work only with one type of icing (e.g., the paper towel on the crusting buttercream). If you know a pastry chef or baker, you could ask if you can watch them for a day to pick up some pointers or see how they do their work. Maybe there are YouTube videos on cake assembly that could help.