Is this a dedicated listening room or a family/living room? Your options may be limited in a shared space. I highly recommend ASC tube traps as the most versatile tool for shaping and tuning the space. The downside is aesthetics for some.
Without spending money, I would revisit your setup. Speaker position and toe in changes of a few fractions of an inch can make a big difference. Seating distance is another thing to consider. Inspect cable routing, keep speaker cables away from power cables.
You mentioned window treatments but didn’t state what type. I like horizontal blinds and wooden shutters. Varying the angle has a noticeable difference in the reverberant sound field.