I trimmed back my Forsythia late last year and I hardly got any flowers this spring. This was the first year I didn't get a lot of flowers. I do believe I did something incorrect. Did I trim back too much, or too late, or what? Any hint (like your article on Lilacs) would be of great help.
When to Trim a Forsythia Bush and How
Certified GKH Gardening Expert
Pruning is usually limited to spring after flowering; however, a light trim in the fall will not harm the shrub. It will simply inhibit the flowering of next season's blooms. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/shrubs/forsythia/forsythia-pruning.htm