My initial thought was Thermal as well. A bit of a look at the current meta. People tend to cover their own Def as well as they can(whatever that might be). Res gets put to the side so offering Res shields can be deemed as more useful. While yes, healing is useful, teams steam roll more than ever so the odds of a team needing a heal is on the lower side. Add in shields from Thermal or where ever, and heals more of a nice to have then a requirement. Also, when a team doesn't feel like they need healing so much, those shields can make you still feel useful.
I know FF got reworked. I just can't be bothered to use it when Time and Cold exists.
The problem for you with Marine is there is a power which literally TPs you into melee, so you either have to skip it or likely deal with going in and out of melee which you said is an issue for you.
For a Defender, I like /sonic due to Defenders getting the most -Res out of it and it is all ranged. Shockwave with a KD IO can offer some nice additional mitigation, and the recharge is decent enough. I have a Thermal/sonic who got on a team with a Cold and health bars really didn't move.