Maybe your campaign is a loss?
Start again and learn from Your previous mistakes.

My advice: Unless a region is liberated, do not gather ressources there. Put 7 or 8 rebels on intel in the region you plan to liberate early and pray for supply runs and advent assaults with at least 3d infiltration time.

For those missions freely spend the intel to maximize infiltration. In the beginning this is the best source of income and those missions will reduce advent forces in the region while also giving you much needed supplies. (Actually, stopping enemy supplies prevents advent to bring in forces from elsewhere, while advent assault reduces advent str. in the region.)

Also; You ultimately will have to defeat missions on higher advent str. The missions leading up to the liberation of a region will bring up advent strength by 1-2 points each. The first HQ attack will probably have no lower advent str. than 5. Staying well ahead on research is the only advice I can come up with. Skipping lasers might be a good strategy.

While advent sterngth is low, it is possible to do supply runs against "impossible" odds, if you are careful. Killing 45 Advent troopers in the first supply run can be a huge boost, both in ressources and experience. Bring roclets and healing though.

Last advice: Keeping a team together under one officer reduces infiltration times and makes soldiers better. Having an officer early is therefore helpful in many ways.

Thank you for your answer.

I did want to wait until every soldier has a laser and a predator armour, and give it a try.
I only waited for the next supplies, but some "unknown" stole my money, damnit...

Is it enough to have a soldier in the haven to find the "intruder" or do i have to scan for intel too?

Only the soldier "scans" for intruders.

Maybe the missions get harder and harder because you are accumulating tactical dark events. I find that the non permanent dark events mod from the "An easier War " series is very recommendable.

thank you!