I'm glad to hear you are making progress with your test assembly and I have really enjoyed be able to participate.
You should definitely pursue the recoil pad solution.
Using spring would only create another problem rather than solving the current one. A return damper needs to be something (like a recoil pad) that absorbs energy; and, springs store and return energy rather than absorbing it.
If a spring were to be installed, it will be compressed by the returning slide resulting in a recoil force of its own that will then partially compress your original recoil spring which will then react and start a period of undamped oscillation between the two springs that will shake the complete test assembly.