Aggressive cat towards other cats

My husband brought Gizmo home in June 2021 as a tiny kitten he found at work, approx 6w old. Goal was to "foster" and find him a home. He stayed. He's neutered and up to date on all vaccines.

Giz and Stromboli were best cat friends. They played hard together and Strom was the " top cat". Strom died in Dec 2024.

Since then Giz has been terrorizing our other cats. He attacks my mom's 4yo cat any chance he can access her. (She lives in an office bc of him).

He chases and tries to attack our 8yo cats.

We are currently fostering a 2yo cat thinking Gizmo needed a playmate who played harder. Ramsey helped some but Gizmo still attacks Brady and the other cats

We've taken him to the vet multiple times. No teeth issues, blood work is fine, no joint issues. We tried prozac pills but he won't swallow them not take crushed in wet food. We tried compounded cream but wasn't seeing improvement. We've changed medications to a different combined cream but Gizmo runs from me anytime I try to apply it and now will rarely let me pet him.

He's anxious in general hence why vet thought antidepressants but I can't keep torturing him trying to get him to take the meds.

Any suggestions?