Current problem
Currently, it is not possible custom slowmode times on mobile, with only presets available. This is not the case on desktop, where a slider allows setting any time up to 6 hours. While bots are a solution to this, allowing you to set custom slowmode times, this should be a native, in app feature.
Proposed change
I would suggest adding a slider or textbox to set a custom slowmode time in the mobile apps, as is possible on desktop and web.
Platform
Android, iOS