In the end I went with POR-15.
Spent many, many hours with a needle scaler and wire wheel on my angle grinder removing any and all flaking rust. Not so bad as it gave me chance to confirm no areas of the frame were really rotted or needed attention, just surface rust & flaking.
I didn’t top coat after applying as didn’t want the shine to remain. After nearly 2 years it’s faded to a chalky black which I don’t mind.
Still do my best to keep it off the winter roads as the roads near me are treated with rock salt.
Same - I hesitate to cover with anything thick b/c I want to see the trouble areas.
I'll likely drill a 1/2" drain hole in front of the LCA brackets too.
~Jason