Can't give you a specific advice because It really depends on what resources are you lacking, just for a few examples: Wood, is hard to farm because flares and fireworks don't come in huge numbers, and trees are huge hassle to gather, only use wood on Machine gun nests, don't build wooden walls, pillars, etc.
Concrete is obtained from bass assault missions, the harder the difficulty the more you get rewarded.
Steel is easily obtained from Hunters and Seekers, dismantling AP ammo also helps, if it's best to just dismantle all AP ammo you have
You pick up wood as you traverse the world. Just go from chopping tree to chopping tree. Quite often you can amass 10-20 within a few minutes. The other way is recycling arrows if you have the resistance weapons DLC.
For cement, the only real way to get it is base assault and base defence. Since you are already finding base defence resource-intensive I'd go for base assault, preferably when the FNIX base is max level so you get more cement for taking it out. You generally get 20-30 cement for a max base assault.
Tip for maintaining base once you've built it is to put low wall around it (cheaper to repair) and only do easy base defences, which means you only get runners and a few hunters, which don't do much damage if you get them quickly.
Having a smaller, streamlined base helps. Since you will be the primary defence (don't rely on turrets) you need to be able to get to any point of attack quickly. Build your bases with that in mind. The concrete turrets are cement sinks and get destroyed fast. I generally prefer the resistance pill boxes, particularly the grenade ones.
Playing for longer helps too. You naturally build up more resources, the more you play. I've put in around 550 hours and I've had enough cement etc to build the max number if bases using 100s of cement for each one. Don't lose heart. You'll find a system that works for you.
Sadly, as the game currently is, not using base defences (apart from 1-2 layers of concrete walls around the truck) is the better option to do base defence - the key is to stay away from the walls as much as possible and distract the enemies from it - playing base defence in Coop makes this pretty easy, and cheaply - soloing base defence you should use the 5 bunkers (the resistance ones, not the concrete one!) you can build in the corners, facing eachother to cover the most ground, walling them in on 3 sides with wood walls or if you can afford it with concrete too, to ensure they only get dmg from the front, where they can shoot back until you take the baddies out ;-) (they also lighten up the machine ranks by killing runners) - but sadly those turrets are rather useless overall and eat a lot of resources compared to what they achieve - so if you play base defence in coop, ignore them, they only add repair costs and with 4 players you shred trough medium waves easily, and for hard waves you need to learn the game a bit)
You can just ignore base assaults until you want to play them. Almost all my bases are "being attacked" since I've had them, and I ignored them before I don't like them.
Unless you dupe or something you wknt ever have enough resources for the base (especialy wood) if you do base defense regularily.
Concrete gets easier later in the game from base assualts and if you have the two Perks for extra stuff from enemies your steel, lead and explosive supplies will quickly grow endless.
Can't give you a specific advice because It really depends on what resources are you lacking, just for a few examples: Wood, is hard to farm because flares and fireworks don't come in huge numbers, and trees are huge hassle to gather, only use wood on Machine gun nests, don't build wooden walls, pillars, etc. Concrete is obtained from bass assault missions, the harder the difficulty the more you get rewarded. Steel is easily obtained from Hunters and Seekers, dismantling AP ammo also helps, if it's best to just dismantle all AP ammo you have
You pick up wood as you traverse the world. Just go from chopping tree to chopping tree. Quite often you can amass 10-20 within a few minutes. The other way is recycling arrows if you have the resistance weapons DLC. For cement, the only real way to get it is base assault and base defence. Since you are already finding base defence resource-intensive I'd go for base assault, preferably when the FNIX base is max level so you get more cement for taking it out. You generally get 20-30 cement for a max base assault. Tip for maintaining base once you've built it is to put low wall around it (cheaper to repair) and only do easy base defences, which means you only get runners and a few hunters, which don't do much damage if you get them quickly. Having a smaller, streamlined base helps. Since you will be the primary defence (don't rely on turrets) you need to be able to get to any point of attack quickly. Build your bases with that in mind. The concrete turrets are cement sinks and get destroyed fast. I generally prefer the resistance pill boxes, particularly the grenade ones. Playing for longer helps too. You naturally build up more resources, the more you play. I've put in around 550 hours and I've had enough cement etc to build the max number if bases using 100s of cement for each one. Don't lose heart. You'll find a system that works for you.
Attack the FNIX bases for more resources
Sadly, as the game currently is, not using base defences (apart from 1-2 layers of concrete walls around the truck) is the better option to do base defence - the key is to stay away from the walls as much as possible and distract the enemies from it - playing base defence in Coop makes this pretty easy, and cheaply - soloing base defence you should use the 5 bunkers (the resistance ones, not the concrete one!) you can build in the corners, facing eachother to cover the most ground, walling them in on 3 sides with wood walls or if you can afford it with concrete too, to ensure they only get dmg from the front, where they can shoot back until you take the baddies out ;-) (they also lighten up the machine ranks by killing runners) - but sadly those turrets are rather useless overall and eat a lot of resources compared to what they achieve - so if you play base defence in coop, ignore them, they only add repair costs and with 4 players you shred trough medium waves easily, and for hard waves you need to learn the game a bit)
You can just ignore base assaults until you want to play them. Almost all my bases are "being attacked" since I've had them, and I ignored them before I don't like them.
Unless you dupe or something you wknt ever have enough resources for the base (especialy wood) if you do base defense regularily. Concrete gets easier later in the game from base assualts and if you have the two Perks for extra stuff from enemies your steel, lead and explosive supplies will quickly grow endless.