There isnt, and shouldnt be, an armor that is so effective that you can shrug off xenos slashes, xenos are a melee only faction and even in extremely close range marines are still at 100% effectiveness in terms of damage, shotties and full auto mk2 for example will hit like a truck even when standing ontop of you.
The main benefit from heavy is that you have a lower statistical chance of not getting fracced, internals to a degree as well but mainly its there to limit the attrition aspect of how xenos break down the marine force. You also get a very relevant increase in protection against FF, saving you a few broken bones here and there.
Currently IMO heavy is in a decent place, the main issue in my eyes is that meds like oxy completely negate damage taken in terms of slow down, its not rare you slash a marine until they literally fall over flatlined, still shooting till the last hit.
The advantage you get from light is just comparatively overwhelming, you get the full speed you want, can give less then a fuck about damage taken with meds, and you die quicker so capping you is more difficult. Any actual attrition damage you take, ie broken bones and possible internals can be mended by the FOB doc, presuming there is one which is usually true for highpop games, IMO anywho.
Realistically if you want the heavy armor to be better, there has to be systematic changes to the combat system, among other things, in order to have it provide actual reliable protection.
Personally id suggest making the mk2 and rifle type guns less effective in the 1-2 tile range, so a close up full auto wont be very effective, blame it on weapon handling or something idk, then you can actually reliably increase the heavy armors slash protection without fear of marines just walking into close ranges and nuking a rav.
Also lessening the effectiveness of oxy, either indirectly by something like adding a few seconds to the checking yourself for wounds thing and removing medhuds from common availability to being back on only docs, or it not reading yourself. You could also hard nerf it with a change to long term useage leading to toxin build up, or having minor hallucinations when taking it, or making you tired etc. but thats just my two cents on minor combat changes that would make armor choices more interesting