A suggestion I have (as a time-blind ADHD person): try to hijack the browsers’ Picture in Picture mode, or display a timer icon in the window bar or tab favicon.
Unless you keep your browser tab open, you can’t see it. A “gauge” icon in the browser bar would be enough to actually be in view without being distracting. Repackaging as a plug-in would give you a tab-free icon.
Screen-level overlays work nicely if you can take the native route (or use sth like Tauri). I have a goat showing up on the screen at random intervals and telling the to slow the fuck down when I speak:
Unless you keep your browser tab open, you can’t see it. A “gauge” icon in the browser bar would be enough to actually be in view without being distracting. Repackaging as a plug-in would give you a tab-free icon.