When it comes to priority, this is how I played it:
The first priority is the quality modules, as you want to improve the efficiency in which you make more quality modules, and other quality stuff.
Second most impactful are the productivity modules, as they greatly improve the production of everything.
Right after that we have the speed modules and assemblers (this includes any producing machine), as they improve the return of investment of the quality productivity modules.
Big priority is also inserters, since they are quite cheap, so making them in higher quality isn't that expensive, and with the beaconed super fast productions, inserters could become the bottleneck otherwise. This is generally very important factor, as quality is a multiplier on price, so making high quality labs, which are very cheap, might be much more sensible to do sooner compared to power armor 2, which is quite expensive on its own for example.
The personal armor/equipment/robots improvements help to build faster, so they are next.
When it comes to the space platforms, everything that is on it can kind of multiply itself to make it more efficient, solar panels, accumulators, turrets (for bigger range when shooting asteroids), producing machines, modules. And here, the beacon can play a big role, as few beacon here and there might be also relevant on the platform, but since electricity is usually a bottleneck, having higher quality helps a lot.
All in all, higher quality stuff related bonuses have different importance, and since beacon consumption tends to be the biggest later on, it might be relevant for big bases which start to produce everything in big quantities.
Edit: And yes, improving the actual functionality of beacons would be too much, as they are OP enough already.
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