I think the idea of strength training sessions for triathlons is more about maintaining speed, ie not slowing down, than it is about getting faster.
For the majority of the 70.3s I’ve done, I’ve felt it is the muscular endurance which has let me down, much like the original poster, it never felt like the aerobic engine was the limiter. It is addressing this issue, which leads me to do 2-3 sessions (of 30mins each) of strength exercises each week.
Like the original poster, I’m rubbish at swimming. I’d agree with the idea of dropping one of these sessions, ie only doing 2 sessions per week - especially if its not coached sessions. If you’re not getting coached, you’re probably not going to get appreciably faster, so getting through the swim with minimal detriment to the bike and run sections, if probably the way to go, and is what I do.
The filler, aerobic bike session, I’m not sure what this achieves really, I’d be tempted to drop this for a session of weights instead.
My “weight training” involves kettlebells and bodyweight exercises. I do some of the bodyweight exercises plyometrically. I’m not training to be stronger per se, but to resist fatigue under load.
Hope helps.
JP