You can retire risk and accelerate progress by launching more smaller missions and accepting that some of them fail.

The public must be enlightened enough to understand that occasional failure ia a much faster and better and even cheaper path to success.

The science done by a bunch of smaller IR telescope is different from what you can do with JWST. (Basically you are reducing resolution but get better time coverage of transients.)

Yes. But you can retire the risk of technologies like segmented mirrors or complex deployment in smaller missions.

These missions don't even need to be physically small.

Then launch the more expensive one.

You can combine multiple mirrors like they did with the black hole photo.

In this case, the black hole image was created with radio interferometry, in which you can separately record electromagnetic waves from individual radio telescopes and then combine them electronically after-the-fact. Visible and infrared interferometry is much more technically challenging because the frequencies are orders of magnitude too high to record in the same way, and so it hasn't really been successfully applied in space. (See the Space Interferometry Mission[0] for one sort-of attempt at this kind of thing.)

[0] https://en.wikipedia.org/wiki/Space_Interferometry_Mission#I...

This requires keeping the baseline constant to a fraction of the wave length. In other words you would have to position the satellites with a accuracy of 100 nm relative to each other. In other words: No we can not (, yet. For a decade at least).

wasn't there an experiment that used laser interferometry to calculate the precise distances between a constellation of three satellites? I assume something like that could work?

That was the LISA Pathfinder mission (https://en.wikipedia.org/wiki/LISA_Pathfinder), the preparatory step for a gravitational wave detector in space. And yes something like that could work eventually. Right now we are doing science on that concept. It takes a decade or three to get the technology mature enough that you can do astronomy with it.

In some StarLink thread on /r/spacex, someone was mentioning that you can't do IR/optical light interferometry without being able to measure the phase of light, and someone else said we're pretty close to being able to.

If you're able to deploy orbital observatories with a StarLink sort of cadence (keep throwing up cheap satellites as quickly as possible), would you think we'd reach more mature observation tech faster? Maybe observing from Earth is dead long term if you drive innovation up and costs down with the observing systems and the supporting data transport networks.

TLDR How do we make decades of progress in years instead?

Being able to iterate rapidly might be helpful to get tech of the ground into space once we have it. But right now even Earth-bound optical interferometry is in its infancy compared to optical astronomy and radio interferometry. It is an interesting subject of research for method development, but we are far away from an instrument that is sufficiently well understood to be useful.

unless the interferometer is assembled in orbit from several spacecraft. a frame plus three (or more) robotic mirrors.

It does absolutely increase risk, which everyone in the industry recognizes - people talk about things like minimizing separation events as a tool for reducing risk. When you're stuck doing risky stuff like this, you can compensate pretty easily by throwing an enormous testing budget at the thing, which is a big part of the ballooning cost of JWST.