Proper buck (step-down) led driver ICs are up to 97-98% efficient. You can't reach that with isolated power supplies since they require transformers and other crap to isolate.
You'd only want to adjust input voltage if you use basic linear regulator style led driver chips, which adjust voltage by dissipating the excess as heat. LED driver ICs of switching type don't have this issue.
In your case, it may be a good trade-off to use only 6 cells and a commercial battery charger off the shelves with cell balancing like this one for example :
https://hobbyking.com/en_us/imax-b6ac-v2-professional-balance-charger-discharger.html which works with either AC in or DC in and has standard JST connectors to plug the battery pack and charge it safely.
So you'd have 6 x 3.7..4.2v = up to 25v or so ... you could use a single step-down led driver for all leds below ~ 21v and maybe a SEPIC (step-up and step-down) for 21v and up leds ... this one would be slightly less efficient, but you won't care that much... you can always have two series of 6 batteries connected in parallel for a total of 12 batteries and you have more current.
You could use for example MP2483 , step-down (buck) or buck-boost regulator (you have example circuits for both in datasheet) :
https://www.digikey.com/product-detail/en/monolithic-power-systems-inc/MP2483DQ-LF-P/1589-1123-1-ND/5298210Up to 95% efficient with ~20v input, but works with up to 48v , over voltage protection, adjustable frequency for better efficiency, dimming, the works.