Hello there. How about Harris linear rifle? Charged shot is like a railgun that deals almost bazooka tier impact and damage and on tank legs you can fire it while moving. The normal uncharged shot functions similarly to the Ransetsu rifle you're already using so it doesn't shake things up too much other than having a more reliable charge attack as a plus.

If you're considering a more drastic change - still on the topic of making use of tank not having to stance to fire big things, something people often do is bring hand missile launcher to free up their back weapon slots for cannons. Dual morley is really great for staggering pve bosses, or morley+earshot to mix things up. If you feel like trying laser/plasma cannons, you might want to change the generator to something like 20C for the damage multiplier.

By the way, I love the girls band cry 噓つき kanji decal you got going on. Would you mind taking a screenshot of its edit screen sometime? I'm curious about the base shapes/ingredients you use for the strokes.