Hello Gregory. Welcome to TT!
I can't speak specifically for the Catholic Church, but I imagine most or all of their teaching is the same as the Orthodox Church.
No, we don't turn into angels when we die. Angels are a different sort of created being. They are the bodiless powers, created by God at some point in the beginning (before humans). That much we should all agree upon.
Of course, some rebelled and became evil. The ones that remained faithful to God are the Holy Angels, and they are fixed in their devotion to Him, having made their decision. We generally teach that there are 9 choirs, or types of angels, and we know some about the differences (not something I've really studied though). Seraphim, Cherubim, Archangels, etc.
Of course God is omnipotent and can do anything. But He does use angels to minister to us, and uses us to minister to each other. As to the reason why, I can only speculate, but it seems to me that the more we do for others, and the more we receive their ministrations, the more love grows in the community of believers. I think the same is true more broadly as well, and God is love, and desires that we be like Him, and love. There's a lot more that can be said about that, but I think that can be a big part of the reason. Another is that we see God doing things in order, with purpose, and there being an orderliness to creation. I think that is a part of it as well.
Interesting questions. I look forward to the input of others.