boltz-backend VS awesome-lightning-network

This would only be possible custodial unless you run an own lighting node. Regarding the cashout to onchain cold wallet you can loop out lightning to onchain with literally any lightning wallet if you use a 3rd party service such as https://boltz.exchange/ or any other service that does the same job. Swapping to onchain can also be done using muun wallet but of course all methods cost the mining fee on top.

watch-only are on-chain wallets, so it will not accept lightning. unless you use muun wallet (custodial by third party) You can create a lightning wallet in bluewallet either by having your own node or adding it from a third party, getalby is a lightning wallet that you can add to bluewallet. After adding the wallet, lightning breez should accept the transaction, to send lightning to watch-only(on-chain) you would have to convert it or pass it to on-chain, https://boltz.exchange either https://learn.robosats.com/ they work quite well to move lightning to on-chain, keep in mind the fees can be high.

Also, look into swap services like https://boltz.exchange and https://fixedfloat.com

Today we’re introducing automation to LiquidOps, an intuitive interface to automate tasks for your nodes with workflows - it’ll restore liquidity in your channels with https://boltz.exchange/ submarine swaps and let you sleep at night!

I use https://boltz.exchange/ to loop out my LN BTC. Works great and it's the cheapest/easiest solution I've found.

I use Robosats exclusively these days and it's easy to send to cold storage using Boltz. Bonus points for boltz because I can increase the inbound liquidity of my Lightning node by sending sats to cold storage with them. You can do the same process they use manually but I've found that the fees are higher and it's less convenient that way

Swap out to a specific address using a service like https://boltz.exchange