Stacking or piggybacking amc7135’s is the way its been done for years. A few times other chips where tried but it’s so easy to just buy loose amc7135’s and stack them that’s pretty much the standard everyone went with. The 105c driver was cheap and programmable with the attiny13. You could scavenge the amc7135 off another donor board or buy them loose at several places. If you want more regulated current just add more amc7135 chips until you reach your goal. There is a limit though. If I remember correctly, the amc7135 has a .2v overhead so as long as the battery loaded voltage is .2v above the led vf at the set current it will remain regulated at that current. Once the battery is below the vf plus the .2v overhaed its running direct drive at what ever the depleting battery can deliver in current at its loaded voltage to the led. Batterys that hold their voltage under high current loads will stay in regulation longer.