Instead of using a hotkey to swap between different bars, why not extend the hotbar at the bottom of your screen to 18 slots? Other mods make this serverside instead of clientside.

Okay but this would be really neat! If I had any experience modding I'd make a standalone client side mod myself

IDK why would you like your own comment but here is a mod which is doing what you are asking for (and from where your suggestion originated I suppose)
https://www.curseforge.com/minecraft/mc-mods/largebar

IDK why would you like your own comment but here is a mod which is doing what you are asking for (and from where your suggestion originated I suppose)
https://www.curseforge.com/minecraft/mc-mods/largebar

That mod is serverside only. It uses the top of your inventory, but trying to switch to the other half of your hotbar gets you kicked from all servers for an Invalid Hotbar Slot.

Good to know, the dev should have stated this in the description (unless it's a bug).