You did the right thing. Editing the transform is the second step to fit+crop the 3440x1440 source into 2560x1440 canvas without distorting it. Usually, you can crop visually by press+hold ALT and drag the border markers of the source, and there are several other transform shortcuts in the menu as well, but manually editing the transform data also works, if you understand what the numbers mean.
If there is no rescaling, only cropping, there is no quality loss. But you recorded with streaming settings with a bitrate of 2500. This is much too low for 2560x1440 video. Go into Settings->Output, change to the Recording tab for recording settings, and change the Encoder from "use stream encoder" to nvenc h.264 (new)". As rate control use CQP with a CQ level of 18-21 (lower values mean higher quality). CBR is only for streaming. For recording, use CQP (or CRF and ICQ with other encoders), which ensure a constant quality instead of a constant bitrate. You can also switch back to simple output mode and simply choose "Indistinguishable Quality, large file size" as quality, and use "Hardware (NVENC)" as encoder.
If you want to stream, configure the streaming encoder in the Streaming tab of Settings->Output. Use CBR with a bitrate according to the Twitch encoding guidelines:
https://stream.twitch.tv/encoding/ (you don't want to stream with 2560x1440, though. Common streaming resolutions are 720p (1280x720) and if you have the bandwidth and have recoding available from the streaming service, 1080p (1920x1080).
