Hello Bold, 3 days and no Community responses must be disappointing.
Your lamb shanks were completely cooked to between Medium-Rare and Medium at the point when you had to stop the process and freeze them. For the most tender outcome you should plan to braise them but at a higher oven temperature. Had you pre-seared the lamb shanks earlier? If so, there’s no need to sear again.
If not, this cook would thaw them, pat dry, season and sear before braising using the broiler of the assumed oven available to you. Turn the meat frequently to avoid burning the meat with its associated bitterness. You could also brown the meat in a 400F oven
If you want the be a little adventuresome, using a paring knife make slits in the lamb and insert slivers of garlic and little pieces of anchovy fillet in the meat before broiling. You could also consider braising the lamb on a bed of Ratatouille or par-cooked lentils with onions and red wine.
You’re the second cook here to use the term dry braise here recently, something this cook cannot comprehend. Braises are moist. Other oven cooking technique terms are bake and roast. Moist cooking will enhance the lamb shanks’ tenderness so just use a conventional braise to finish your lamb shanks. You can return to the SV technique next time.
Most tender? Sufficient?
Where i live most tender means the shank meat needs to be tied on the bone which i consider to be a culinary abomination, but then i still have all my teeth. If that tender is your objective then conventionally braise away for another 3 hours, maybe a little more, but at a higher temperature, say 275F in a covered casserole or Dutch oven.
To my taste, braising for 1 1/2 to 1 3/4 hours would be sufficient for your par-cooked lamb shanks.
To be sure you might want to give the meat your good old fork test to judge its doneness to your liking. Stop cooking only when it’s sufficiently tender.
Do well and enjoy.