Just wanted to clarify when you say its not recommended to groom onto the rigged character but rather the final animated character, are you able to expand?

Correct. You can either:

1. Add the hair on the final character with baked alembic animation.

2. Or, groom the hair on the T-Pose, low-poly character and then transfer it to the character with baked alembic animation. 

I've noticed I get better viewport feedback disabling everything from "Hair from guides" downwards, is it ok to temporaily disable these and turn them back on when the animation is done?

Yes. That is acceptable. 

The other workflow I wondered is export the hair as alembic, rig the character, animate it and then re-apply the hair via alembic import (then add any physics if I need to but I'm thinking I probably wont need them).

Correct. When the hair is approved, export it as alembic and attach it to the animated character(The position of both objects should math for the attachment process to work).

Jeordanis Figuereo (Product Designer. EPHERE Inc.)

I do have a question regarding "2 - transfer the hair from the t-posed character to the animated character": Would I export the hair as alembic from t-pose, and also export the animated character as alembic, and then combine them?

Correct. Export the hair as Ornatrix Alembic. Then import it in the scene with the animated character(it should have at least one frame in T-Pose). The hair will import as an Ornatrix hair object with a BakedHair node in the stack. This node have an option to attach the hair to the character. Another option is to use Ground Strands. 

I'm wondering, is there any documentation or tutorial covering hair for animated characters? I've been going through the docs and they seem more settings specific rather than explaining how to use it in production, maybe that will come more with experience for me.

I don't think we have one for this specific case but is a good suggestion for the future. However, many people use different workflows. This is just the one I have used in production. 

Some artist groom the hair in a separate scene and reference it into the animated character scene.

Others export just the guides as Ornatrix Alembic and rebuilt the groom on the animated character.

Others animated the character with the hair already on it. In this case with a different version of the groom that is less dense with thicker strands, converted to polygon meshes. 

Others use proxies instead of alembic. 

Jeordanis Figuereo (Product Designer. EPHERE Inc.)