many things to do
main mission, unlock safe house,takeover control points
unlock these sites allow fast travel, also control points may give weapons too
for solo need to add skill to assist, assault turret is a helpful skill. for armour repair, fixer drone, hive, chem launcher also useful
roam open world, loot resources node (water, food, components) & donate to control point adds xp too. not much but it adds up
there also smaller missions like rescue hostage, stop execution, supply drops, territory control … tp earn xp. open up map to find these challenges
i read best to do side mission later once you a few level above the side mission, earn more xp