MP4/MKV/WebM/MOV are just containers - what codecs are in your input and output?
I’m fairly certain kdenlive supports x265 for H.265 encoding - that will be one of your best choices as far as quality vs. size tradeoffs. Not sure how VP9 or AV1 will compare - last I checked AV1 was slower and often not present in distro-provided ffmpeg builds, but it’s theoretically not patent encumbered and should perform similarly to H.265 in bitrate/quality tradeoffs - AV1 - Wikipedia
Last I checked it was really difficult to use hardware encoding with kdenlive, but it’s been a while since I gave up on it in favor of Davinci Resolve.