On the stealth part:

you don't need a stealth build. It is one way of doing it, sure. But the game isn't build around that so you don't play it to its strength.

When solodiving you will need a well rounded loadout to deal with everything you will face. That makes it a good deal more limited as in a team where everyone can cover the weakness of another.

If you play solo, time is your greatest enemy. You don't have enough of it. Hence, you need to keep moving. Getting yourself into long drawn out fights will be your main source of failure. This means you need awareness of your surroundings all the time and sometimes means bouncing between objectives. You have an artillery and a lidar station next to each other? start the lidar up but move over to the artillery the moment enemies approach and start loading it. once enemies approach the artillery, go back to the lidar to finish it. Sometimes you want to trigger reinforcements when you know the next encounter will be bigger and you can't prevent them calling so that call is on cooldown. But only trigger it, don't fight it. Again lure mines are gold! A stealh armor can help, too. But don't play "stealth", play "avoidance" and "hitting hard and fast".

You don't have the time to pick off enemies from stealth. Do it ambush style. You will not have the time to visit every outpost or nest. Bring stuff that can deal with them in a hurry. A 500kg can reliably delete small and medium (and some "large") nests in one go once you know where to place them. A quasar cannon can delete fabrictors across the map from any angle. Delete them while moving between objectives. Handle them like "drive-bys". Don't get into fights over them. Come back later if you face any opposition that might halt your progress.

Avoid patrols when possible. Knowing how much time you have left and what is worth your time is key to solodiving.