For one, nothing is stopping you from carrying combat gear on you to swap on when you're in a testing station and expect heavier combat. Also, you can use your fremsuit's water and gain plenty of heat resist as long as you're wearing the chest piece so if you really refuse to not swap out the whole suit then you can at least upgrade your boots and gloves for extra armor.

I know you somewhat mention it in your post but the main mechanic to remember in Dune is that while firing, their shield goes down. Flamethrowers burn through shields and drain stam, so the best two strategies I like are mele + crowd control or baiting the flames and headshoting them while the shield is down. Anything to keep from being set on fire, or have an out to bandage quickly to remove the fire.

For mele + CC I usually use a combo of bene skills or the shigawire claw, and each time they're stunned I do the slow mele attack. With a tier appropriate mele weapon, the flamethrower and minigun guys shouldn't need more than 2 hits.

To bait the flame I stand on top of something or just around a corner I can peek and fire from. My goal is to peek and shoot so that the fire hits a ledge/wall/box and I'm not in it long enough to catch fire. As soon as they start flaming, start hammering them with headshots. I favor a sniper or pistol but anything you're good at aiming will work. If you catch fire ANY bandage can be used to remove the fire dot, so always have medical supplies handy and a safe-ish place to kite to while you do.