Feeling stuck and discouraged

Hi everyone,
We’re looking for some advice because we’re approaching 6 weeks of introductions and feel like we’ve made almost no progress.

Our resident cat, Trout (about 4 years old), and our new cat, Minnow (about 4 years old), are both neutered males.

We’ve been following a slow introduction process. Minnow has his own room, and we’ve done scent swapping, site swapping, feeding on opposite sides of the door, treats together, Churu sessions, and gradual visual access through a baby gate.

The confusing part is that they often seem completely fine together. They’ll touch noses through the gate, eat Churu side-by-side, calmly observe each other, and don’t seem fearful during those interactions.
The problem starts when the food or distraction is gone. Trout becomes intensely fixated on Minnow. He’ll stare, lock onto him, tail twitch or thrash, and try to approach him. Sometimes he’ll hide on one side of the wall almost waiting to pounce if Minnow would walk out(even though he can’t and there is a gate). Minnow usually retreats or hides when Trout comes into view. If Trout gets too focused, he’ll sometimes lunge at the gate. We think Minnow may have even gotten a couple small scratches through the gate despite Trout’s nails being trimmed. Trout acts perfectly normal outside of seeing Minnow, his behavior and habits are not affected and he isn’t agitated.

Minnow recovers quickly after these incidents and goes back to normal, but he’s definitely wary of Trout and tends to hide when he sees him.

We’ve tried:
Scent swapping
Site swapping
Feeding near each other
Churu sessions
Daily visual exposure through a gate
Nail trims
Calming collar on Trout
Keeping interactions short and supervised

At this point we’re struggling because it feels like we’re stuck in the exact same place we were weeks ago. Trout’s fixation doesn’t seem to be improving, and Minnow is ready to be out of his room and share the house with Trout.

Some questions:
Does this sound like territorial aggression, play aggression, or something else?
Is it normal to still be at this stage after nearly 6 weeks?
Should we continue gate work, or are we missing a step?
Has anyone had a cat who seemed friendly one minute (nose touches, eating treats together) and then immediately became fixated or aggressive afterward?
At what point do you consider consulting a behaviorist or asking a vet about medication?

We’re feeling discouraged because we really want this to work, but we’re worried we’re either moving too slowly or doing something wrong.

Any advice, success stories, or suggestions would be greatly appreciated. Please be kind, this is our first cat introduction. Thanks!