Those "Pathfinder" weapons are sure good at giving you a headstart early in the game... but they are not the best weapons in the game. There are more interesting ones that can be unlocked. The very best ones are those you craft yourself, especially if you give them certain augmentations during the crafting process.
What I did:
As soon as the game allows you to unlock Cryo pods (... depends on your AVP score and the viability of the planets you build outposts on...), please unlock this one:
Military Pods > Reconnaissance
The result of this will be that every planet's surface you visit will be covered with 'hidden' containers. Well, not really 'hidden' in any way because their location will now clearly be visible on the map. All you need to do is get into the Nomad, drive to those locations and then fetch the loot. Some of those containers are near or even inside Remnant sites (there might be a few hostile bots), or on high cliffs (you need to use your jetpack to get there...) or sometimes in caves. But totally worth it. I got tons and tons and tons of good guns, good armor and rare materials from these containers!
Then, as soon as you can either research or buy this augment mod, please do it: "Vintage Heat-Sink".
That augment totally eliminates the need for ammo crates - you get infinite ammo. Yes, your guns might overheat here and there, but you can stay in cover while they cool down and regenerate their ammo, no need ever to run out of cover to find an ammo crate.
Next, research this Angara sniper gun: "Naladen". When you craft it - give it the "Vintage Heat-Sink" augment!
Why this gun? It does massive explosive splash damage to anything you fire at. It's like calling in an artillery strike. And with the "vintage heat-sink" you will never run out of ammo, you just have to wait a few seconds for the gun to cool down again. Combine this with consumables like "Incendiary ammo" or "Disruptor ammo" and skills such as "Concussive Shot", "Incinerate" or "Throw" and you'll be setting off explosions and combo detonations all over the map! Even large space monsters or large groups of enemies don't stand much of a chance.
Then spend some "Andromeda Initiative" research points and research this assault rifle: "Revenant".
Why this gun? It deals a lot of damage, it has a high fire rate and has a very large clip capacity. It was the very best gun to have in "ME2" and was still very good in "ME3".
Apply the same trick as above: When you craft this gun, give it a "vintage heat-sink" augmentation. Taddaaaa. No more ammo problems with this gun. Add a scope (... you will run into so many containers everywhere, I am sure you will sooner or later find a suitable scope! ...), add a large clip (... can be obtained from containers, traders, dead enemies ...) and / or a barrel for extra damage. With the right skills and profiles (e.g. Soldier, Infiltrator, etc.) you can pretty much eliminate recoil and stability issues.
Now, armed with these beasts of guns ... do I miss not being able to further upgrade my "Pathfinder" guns? Nope, I in fact don't :D
I know, I know ... this is not a "solution". But maybe an acceptable "workaround" to help you beat the game? 🙂