Best way to manage multiple messaging accounts on desktop without a dozen windows open?

The thing that fixed this for me was moving to a single desktop app that holds multiple accounts of the same service at once, instead of one browser tab per login. I use Spanbox: two Slacks, two WhatsApps, Telegram and Discord all live in one window down a sidebar, each its own real session, one unread count. Free and local, so nothing routes through a server. Before that I tried the pinned-tab approach and it never held up. Browsers throttle background tabs, so notifications lag. A dedicated app that keeps each session warm was the difference.

Rambox does the multi account sidebar thing too and has been around a while. Give a couple a try. The right one really depends on which services you need and whether you want it local or not.

Workspaces on a second monitor plus one app that stacks the accounts. Stopped alt-tabbing entirely.