I would argue that a flash hider ('bird cage') is the least functional accessory on a civilian AR. It's function is to mitigate the flash so the enemy is confounded when trying to locate your position. It has other functions like minimising flare if an NVD is clipped-on and the weapon used at night, but that is a niche need. In an 18" barrel, most powders will be 100% burnt by the muzzle and so the effect of the hider will be small. If your Grendel is intended as some sort of long range human conflict scenario then sure, get a bird cage (they're cheap anyway). Or, if you think it 'completes' the gun as far as how it looks then it's de-rigueur to have a hider. As for a hider doing a good job hiding your location, it is arguable as to how useful they are in doing this given all the other reasons why shooters are seen and located (shape, shadow, sound, silhouette, surface, spacing and movement).
I don't know the crown they put on Odin barrels but I am assuming it is not just a straight right-angle parting. If it's a right-angle parting you will need some type of crown protection. A bird cage or over-long thread protector will do this. If however it has any type of typical muzzle recess, for example an 11" crown it will be like any of the millions of hunting rifles being dragged around the countryside - you won't need extra crown protection.
A bird cage will make the barrel slightly longer and heavier compared to a thread protector, but not by much and you probably won't notice it.
A suppressor or brake will have more functional use than a bird cage but if it's just between a cage and a thread protector then I would choose the thread protector.
I don't know the crown they put on Odin barrels but I am assuming it is not just a straight right-angle parting. If it's a right-angle parting you will need some type of crown protection. A bird cage or over-long thread protector will do this. If however it has any type of typical muzzle recess, for example an 11" crown it will be like any of the millions of hunting rifles being dragged around the countryside - you won't need extra crown protection.
A bird cage will make the barrel slightly longer and heavier compared to a thread protector, but not by much and you probably won't notice it.
A suppressor or brake will have more functional use than a bird cage but if it's just between a cage and a thread protector then I would choose the thread protector.