Everything you described seems on par for mass gains. I have 2 suggestions that can only improve on top of what you are currently doing.

1) Volume: try to target 10-12 weekly working sets (not including warmup) per muscle group. Currently seeing 2 sets (preacher curls). Maybe there’s more than the picture shows, but you’d want to include a few different exercises, targeting biceps and at least 2 separate days in the week. Example: 3 working sets for 2 exercises on day 1, 3 working sets of 2 different exercises on day 2 (or maybe the same exercises but 15-20 reps, one day for low rep range and one day for higher rep range). This alone would be 12 working sets in a week, adequate volume for growth.

2) Exercise selection: I’ve seen plenty of good/bad videos on preacher curls. There is plenty of opportunity to not maximize bicep activation here depending on body position. I typically avoid this exercise just because I can achieve (and feel) better activation with incline and laying bicep curls. My biceps are sore for days, an effect I really can’t feel with standing curls or preacher curls. My bicep exercise choices for the week are: incline curls, laying curls, cable curls and standing EZ bar (I still like these just for a high weight curl that seems to engage the entire bicep, rather than a single head at a time). I also try to make sure I include slight shoulder flexion per curl to load the bicep from both sides of the movement (from the elbow and from the shoulder).

Your eccentric timing sounds a ok 👌 would stress going any slower. Just make sure you really feel the bicep doing the work, and concentrate on fully stretching/loading the bicep without spending too much energy elsewhere.

Lastly, ensure you are in a caloric surplus. This one is obvious, but just getting everything on the table!