I may merge with with one of the existing ones that parallels your request:
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Continuous Press, Quick Intensity Adjustment [Feature Request]
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Feature Request: Turn intensity up/down in % blocks in addition to % points
Related to the first link above, the “continuous press” is supposed to work on some platforms. It does not apparently work for me on PC for some reason.
If you are on a regular bike & smart trainer (not a smart bike), you can consider using shifting just like we do when switching from seated to standing when outside. Here is an article I wrote specifically about ERG and shifting to stand (or any other time you want to lower your cadence quickly without waiting for ERG).