In my opinion, (a 4 year sufferer) if I'd have known back then what I know now, i'd have gone for the surgery a lot sooner.
As you've said, you've tried reducing your miralax intake and it hasn't fixed the issue, I see it as the miralax is helping you deal with living with a fissure and Suzy is spot on, the scar tissue had an affect on the healing. So it's likely that the fissures are still there but won't heal.
It's difficult for anyone to give you the answer without knowing the full extent of your fissures.
My CRS suggested an examination under anaesthetic after seeing it was too painful for him to do an internal exam and would decide on the procedure once he'd had a good look. This is when I had a double fissurectomy to remove the scar tissue and make it fresh, then he gave me Botox.
If your fissures aren't able to heal, then I guess Botox wouldn't be too successful other than making a BM comfortable for a few months. I would suggest another conversation with your CRS to discuss how chronic your fissures are. If they don't know, then how can they judge without an examination planned?
Hope that helps
Welsh x