Yup love the necroposts and people always want more. They call deliberate design decisions of ZOS as broken items for inventive Add-on to fix.
In a simplistic form, the API for the active ability bar is a fixed size integer array of six items. One of those is for an Ultimate. It becomes two arrays when you reach high enough level. Technically there is a third when certain Ultimate are active. The thing is only one array/set can fire/execute off one of abilities at a time. All the rest are blocked/inactive. There is no bypass available. You can switch between bars but abilities themselves can only be changed outside of combat.
The comments about the potion bar work because that ‘array list’ is read/write, can be changed during combat, and the effects override each other. Combat abilities are temporal and may even stack on the target(s). Hence one of the reasons ZOS locks them.
An Add-on could fill the screen with buttons for all the abilities but it would be fairly pointless because the six slot ability bars are read only during combat. There are a couple decent add-ons which help with configuring and swapping abilities outside of combat.
You have five abilities and one Ultimate active at any one moment. Mix and match and leverage the bar swapping for different situations or be the dull one and one button spam the FOM ability. Two-hand Wrecking blow and dual weld Whirlwinds are popular right now.
