In terms of lack of launch on demand side. One thing that would solve this almost immediately is to re-purpose money from legacy science / launch efforts.
For example, take 4B/year from SLS. You could fund just an incredible amount of activity with that.
Correct. The math is actually about 500 electron launches.
That is to replace the cost of launching one SLS. If you include development costs cancelling out SLS now would easily save $4B per launch + 10B+ of dev costs.
I think the proposal would be to open up funding for both additional launches AND additional science missions. For the let's say $6B/year or so, you could do a huge amount.
F9 Heavy can do a lot of good science missions, and many payload sizes are coming down. I'd personally include the Ariane stuff as well, James Webb launched on a Ariane 5 (around $200M per launch - so can get 20 launches on that or 10 launches + 10 new $200M science missions for one SLS).
This ignores the DEVELOPMENT cost of SLS which is mind bogglingly large as well as some of the ongoing just sustainment costs (also insane).
For example, take 4B/year from SLS. You could fund just an incredible amount of activity with that.