I'm not sure I have much to offer but DD is 15 weeks and I'm just back to work so wanted to figure out an alternative to feed-to-sleep.
To be honest my approach was so mish-mashed I don't know if it even counts as an "approach"!
I started with a classic bedtime routine (hadn't had one before)- PJs, song, feed, bed. Up till then she'd have done non-nutritive sucking for ages before finally conking and falling off boob. So instead I removed boob when she finished the feed proper and then did rocking/walking in my arms and shushing/white noise. When she drifted off I'd gently put her in cot with some white noise on.
Initially she'd wake instantly on hitting the cot, start protesting, and I pick her up and start the rocking again.
This went on for about a week- on nights she was particularly resistant I would "give in" after a bit and boob her again. TBH I was really stressed by it and it made bedtimes reallllly long. And even though it was a theoretically "no cry" approach, in reality DD did often cry in my arms as I tried to calm/rock/cuddle.
So I decided to forget about it after the week or so and just feed her to sleep again. BUT I did the Pantley Pull off (very gently and gradually) and then I'd put her in cot with a hand on her chest and more often than not she'd settle- if not, back to boob. I think the rocking to sleep week, though exhausting, did give her some alternative to boob to sleep.
I started this all around 11 weeks I think because she'd stopped cluster feeding by then and seemed ready for earlier bedtime.
Now, we do bedtime routine, feed till lightly asleep, then into cot with white noise (Ewan the sheep) on as a little prop for her. She's now beginning to master thumb-sucking but the dummy is an alternative (something she used to reject in disgust but I kept gently offering regularly and she'll now take it long enough to nod off)- one or the other and a hand on her chest and I lie my head on her cot side and close my eyes. She usually wakes when put in cot but now, hallelujah, will shuffle a bit, look up at me for the odd sneaky smile, and slowly drift off to sleep.
Seeing that written down makes me so proud of the little mite!
So it was sort of a mix of what I considered a failed week of trying to rock to sleep, a "fuck that" and return to much lovelier (for us both) feed to sleep but with a few gentle changes (bedtime routine, pull off, and white noise), and now a really lovely, cuddly and happy routine that works for us both.
Eventually I'll push the feed back and make it more separate from the sleep bit but not till she's older and more ready.
I do think DD was a bit young during the rock to sleep week (reading stupid sleep blogs was what made me think I needed to change things!) but in hindsight I do think she then transitioned into a more flexible stage then that allowed some softly softly alterations to the boob pattern.
To be fair though she wasn't quite as stuck to boob as your LO but maybe you might find some of this rambling, massive essay helpful? I can't believe I'm giving advice really as I feel I totally spoofed it! But I do now like bedtime again and as PPs have said it's really allowed me to get back to cherishing DD and our lovely nighttime cuddly feeds