I have an almost identical situation. I'd been on finasteride for about 15 years. Noticed increased daily shedding about 14 months ago, when I was 39, so feared that the meds were finally losing their efficacy. I switched to Dutasteride in March this year and have been on it for 9 months. I still notice crazy shedding to be honest. I probably catch about 40 - 80 hairs in the shower (plenty more go down the plug I expect), but my hair seems to be a permanent continuous fall. It's mostly long, thick hairs, but I also notice a few small ones.
Very hard knowing what to do next. I don't know if there's any evidence to suggest that meds lose their efficacy as you age. However, other than gradual thinning at the crown, I'm not noticing a huge amount of change on my head. I begun losing my hair at 18 so have had a long history of hairloos and I've done okay I guess.
However, it's so hard to compare pics with my hair - I've got long, super fine hair. I can take one pic and it looks good, and take another pic and it looks super thinning.