I made a quick mockup for another idea, basically having only two sidebar “header” tabs.
One for the regular sidebar and one for custom, it’ll probably take a bit of space but it’ll give more flexibility as you can put more addons there or when there will be the possiblity to pin or drag & drop favourite panels and with the dropdown list added it’s even more flexibility ,similar to what other Softwares like Maya have.
PS; we could even add icons to both tabs so when the panels size is small only the icons shows up, something like how the toolbar does it.