https://addons.mozilla.org/firefox/addon/auto-tab-discard/
I’ve got more than 30 open tabs, though in practice I don’t actually need ALL those tabs loaded. The extension unloads inactive tabs after a configurable time. You can also configure the extension so that pinned tabs are not unloaded, certain domains/URL patterns are not unloaded, etc.
It’ll be absolutely fine to use your work device for personal stuff. Until they want to fire you and need a reason for it. A quick audit of your usage, then bye bye!