Right now many players don't play raids. The cost of raid ressourses are very high because of that.
Some people thus do the raids for the value of those ressourses but many find it a chore.
I think that the main problem is simple: Long + Nonchallenging = Unfun.
Personally I love both PVP and Invasion and I loved chase before it was removed. And I think the reason is simple:
In all of those you can go straight to the target, try to destroy it at fast as possible while calculating risk to avoid dying in the process then go to the next. Targets represent a great challenge and meaningful choices: For chase, destroying the escort cost time but allow more safety. Same for waiting a few seconds for the weapons to overheat. In invasions and PVP you have to assess what the vehicle can do and try to find a weakness or a good angle of attack.
The huge problem in usual PVE maps is that they throw at you loads and loads of high health low menace minions. It's even worse at high PS because to "increase the challenge" parts have their health bumped up without changing much the base model. It takes multiple seconds of focus fire to destroy a single minion. And those aren't really a threat so it's just cleaning.
So my idea:
Make all PVE map have objectives that can be done fast with risks or slower and strategically.
-Remove all protect missions. Protecting a target is boring and frustrating. In case of moving targets you have to move at their speed. In case of a static one, waves are a really frustrating feature because you need to clean after each one (particularly those stealthy turret guys that just flee all the time)
Alternative: Maybe a complicated one to code but steal the truck could be fun if one of the players has to go in a circle at the start and actually take control of an (armed) truck. All the players could do it to avoid problems but any truck reaching the end would be victory. Of course keeping a few player with their normal car would probably be more efficient to defend the truck(s) (trucks are not heavily armed) but having 4 huge trucks try to race to the end could be fun if people want to do it that way (they may lose often). There could be a few gates that require destroying but other than that the main threat should be mobiles enemies attacking from all side that you can either try to outrun or to destroy as well as fortified static defenses and minefields.
-Tower capture: Again the waves are a problem.
Solution: Remove the generator part, only have the 2 towers. Both activable from the start. Towers would work like capture points (with smaller radius). Players staying near them would capture them. Both towers captured=win. Having a player capturing a tower would trigger a constant spawn of enemies that would rush to destroy that player. It's possible to capture both towers at the same time and multiple players inside the same capture zone would make it go faster. Enemies would also spawn faster. Each 1/3 capture would trigger a leader spawn to spice things up. The goal is to have a very fast capture time if you stay in the zone but the focused fire of a leader and multiple minions will probably make that hard. Leaving the zone to stop their spawning and deal with them before there is too many of them is a viable strategy.
-Steel cradle: Not horrible but the turret destruction and generator activation are both a bit tedious.
Solution: Have 3 leaders vehicles on the map (and a few minions to protect them). The leviathan slowly gains HP from 25% to 100% as long as those are alive. When it reach 100% or the 3 are destroyed the cage opens. If you are very slow you will face the leviathan while there are multiple leader vehicles still on the map. If you go too fast to destroy the leaders you may face a damaged leviathan but you may be damaged as well due to recklessly fighting the leaders.
Those solutions aren't the best, just ideas to push in a direction: Having faster raids where you can actually die if you go too fast. People want (or at least I want) a bit of pressure on them and managing risk is a good way to do that. If raids wake people sweat a bit like in PVP they will be played and liked.