I think that will be the future. Phones will get better and the niche for dedicated cameras will get smaller and smaller. I am also thinking about getting one of the next iPhones and then using my X-T3 only for things like wildlife where the phone can't (yet) compete with a big zoom lens.
I have had both and I think the X-T2 was a big step forward in terms of handling. Focus is better and the dials are better. Just about everything is much better. As far as IQ they both are different and have their strengths and weaknesses.
Step up rings will work well. The smaller size filters are also cheaper so if you use one lens a lot it may be convenient to buy one that size. It can be annoying to have a 77mm on a 58mm lens.
I am on it and I am always surprised how pictures I am not proud of get 50 likes and pictures I really like get only 3. It's definitely a learning experience what people like. Still haven't managed to get 100 likes or more.
No idea when it will happen but computing power tends to win over hardware so I think mirrorless will take over in the next few years. However, I think mirrorless cameras will also be replaced by other devices like phones.