To adjust the stats of a weapon, I think you need to do it via post-hooking the appropriate function in lib/tweak_data/weapontweakdata

In the case of the Welrod, you'd need to do:

Hooks:PostHook(WeaponTweakData, "_init_welrod", "UNIQUE_HOOK_ID_init_welrod", function(self, weapon_data, tweak_data)

	-- adjust the stats of the welrod here.
	-- see vanilla code here: https://github.com/steam-test1/Raid-WW2-Lua-Complete/blob/master/lib/tweak_data/weapontweakdata.lua#L4227

end)

and then in supermod.xml, remember to hook this function to lib/tweak_data/weapontweakdata, like so (and adjust the other mod definition values):

<?xml version="1.0"?>
<mod name="cool modded welrod stats"
   ...etc
>
	<hooks>
		<post script_path="LUA FILE NAME HERE.lua" hook_id="lib/tweak_data/weapontweakdata"/>
	</hooks>

	<!-- here's how you do auto-updates <updates>
		<update provider="modworkshop" identifier="NUMBERS AFTER /mod/ IN MWS URL GO HERE" important="true"/>
	</updates> -->
</mod>

Disabling/adjusting the animation which plays between shots would probably require a lot more finagling around with animations (which is a whole other can of worms), but this method should at least allow you to make it function at the desired speed.