Dog a bit aggressive on leash but not otherwise?

Hi yall!
My six-year-old female Frenchie Fish (only fish I ever caught ahah)we are Apartment based

This last year she was attacked by another dog about her size while on a leash. The attack was basically she got tackled & bit on the neck. She didn’t have any blood drawn for context and I scooped her up before anything else could happen! Not discrediting it but for context!

Now I feel like she’s fine on a leash when we go for walks and when we go outside to go potty. The problem is starting when we approach another dog on leash. If she’s off leash, she has no issue with any other dogs or animals in general! or any other person really but when she’s on a leash, she seems very guarded and aggressive towards anyone else or any other Dog.

I’ve tried redirecting her focus. I’ve tried training treats.

I’m considering getting a trainer, but I wanna resort to that last if I can help it. Any comments, advice programs, YouTube videos anything is very appreciated.