The hold invoice stuff does not really need any customization to lnd really. Last time I checked Zeus's code it was pretty close to vanilla lnd. Same as Blixt.

Also, BOLT12 is in the pipeline for lnd, so that's not true.

What other stuff are you referring to? 
Lnd is first out with Taproot channel support, which is great for privacy, which Zeus and Blixt now ship with.