I've never had a soil test done, so I'm not an expert but here's what I'd do.

Test the soil now before adding anymore amendments. That way you'll know if your soil contributed to your garden doing poorly. If you amend it now, and test it in the spring, you'll have changed the conditions and your soil test then may not indicate what your problem may have been this year. How much does a soil test cost? If I were really curious, I'd test now, amend accordingly and then have it tested again in the spring to see what difference it made.

However, someone else may recommend something different.

Having a soil test done in the fall means that you can make the necessary corrections this fall and they will then be in place next spring when your plants will need them. Having soil tested next spring means that any adjustments you make then probably will not be of any benefit to those plants until about the time your growing season ends next fall.