Query replace a copied string by another copied one

Question

Use case: I want to:

1. copy a (long, not handy to manually type) newstring,
2. replace all occurrences of another (long, not handy to manually type) string by newstring.

Current process (not handy):

1. Copy (mark + M-w) string in the (main) buffer.
2. Start the query replace process, using copied string (M-% C-y <RET>).
3. Select another window (C-x o) to switch from the minibuffer to the main one.
4. In the (main) buffer, (optional: search (C-s) for newstring and) copy (mark + M-w) newstring.
5. Select another window (C-x o) to switch from the main buffer to the minibuffer.
6. Paste (C-y) newstring and continue the query replace process (<RET>).

Possible start of track: This answer provides the a nice way (C-s C-w) to search for the word after the current mark.

Question: I wonder if:

1. the previous track could be extended to replace for the word after the current mark,
2. or if there is another, better, track for my use case.

Answer 1

Avoiding window switching:

1. Copy string
2. Copy newstring
3. M-%C-yM-yRET
4. C-yC-u-1M-yRET

Or if you can get used to copy newstring before string:

1. Copy newstring
2. Copy string
3. M-%C-yRET
4. C-yM-yRET

And another option is to write function which will call query-replace with most recent kill ring entries and bind it to some key. Then just:

1. Copy string
2. Copy newstring
3. key for my/query-replace-function