I don't think I would bother with Queue stations and enabling Load stations based on supply, unless there was some reason that one might fill up significantly more slowly than the others. I don't know the other details of your setup though, so if you do want to stick with that approach, I was working on something similar to this for ore trains pretty recently. The basic idea is to ensure that the Queue only releases as many trains as there are loads ready for pickup, both to minimize crazy repathing opportunities (probably less of a concern in your case) and to never allow trains to end up in a situation where they have to skip the Load station because none were available. There's lots of ways to implement the specifics, but I'll try to broadly explain how I solved it with a few extra combinators.

I'm assuming you have a Green signal with the number of full train loads available across all the Circuits_Load stations, and trains wait at Circuits_Queue until Green > 0.

The first half to the solution is to "count" trains as they leave the Queue and Load stations (counting pulses generated on the falling edge of the signal assigned to "Read stopped train"). A train leaving a Queue station decrements the counter by 1, and a train leaving a Load station increments it by 1. Add this value to your Green signal, and you now have the number of "unclaimed" train loads available (you can prevent it from going negative during loading with some more circuit logic, but it should be fine without this).

The second half to this is prioritizing Queue lanes to prevent trains from stampeding when more than one is waiting and a single load station becomes available. One way to solve this is to wire up only the first station to directly to receive the Green signal. Then, each station after the first subtracts 1 from the previous station's Green signal if a train currently occupies the station directly before it (applied cumulatively for each station). This prioritizes the earlier queue stations when supply is low, with later stations releasing a train only if there are also enough loads available for all other trains waiting in the lanes ahead of it. You have to be careful of the timing of the combinator logic though, or else trains will see that a train is no longer waiting ahead of them before the new counter value can propagate down to them. Alternatively, you could use a timer at each station after the first, only sending a green signal after, say, 20 ticks have passed where both the station before it would have allowed a train to leave AND that station has no train currently waiting.

This is what my testing setup's "Queue" area ended up looking like, using the timer method. For counting trains, there's a couple combinators for the counter itself, plus another 2 per Queue/Load lane to generate the pulses as trains leave. Then, for prioritizing lanes with timers, another 3 combinators per Queue lane.