I'm wondering how to approach replaying the SP campaign, to gather collectibles, etc. I'm wondering if, having finished the campaign once, I should start it again or just cherry-pick individual missions? If I restart the campaign from the beginning, will I have my full complement of Titans for the mech battle sequences or must they be re-earned, as in the first run? Many thanks.
If you restart it all the way you lose all weapons and titans you have gained or unlocked. Like the first run over again, you also lose any collectible progress, if you don't care about the collectibles and just want to play through it again it's definitely worth it. Also after playing a second time you can come to your own conclusion on the best missions to replay if you wanted to.
As far as I can tell, your collectible progress doesn't reset. Any collectible I got on Easy isn't there when I play through on Master. However, everything else is like playing the Campaign all over again (you don't keep any Titan loadouts).