Saying this in the nicest way possible, this is 100% skill issue. I have been playing the M109 quite a lot since it’s addition, so hopefully I can help a bit.
Firstly, stop aiming center mass or aiming for the first flat surface you see, it will more often than not result in 0 damage, other than maybe an MG destroyed. Despite it being a funny HE slinger, you do actually need to aim. Most times I get 10+ kills a game I’m aiming for gun mantlet, turret fronts, cupolas or side armor above the track but below upper hull. Sometimes you will get a kill by just shooting someone’s track, really just depends how light the tank is, stuff like M41s, Kpz. 4/5, etc.
Secondly, and this is more of a point than helpful information, depending on the M109 you use, the reload is either really good or its only a second or two slower than other howitzers of similar BRs. The M109A1’s have a 13.3 second reload flat, regardless of crew level and qualification, as it has an autoloader/assisted loader, where as the base model M109’s I believe start with a 17 second reload but can get down to close to 14, if not below with a maxed crew.
Thirdly, it is in fact quite speedy, you just need decent level driver and all the mobility upgrades. This is nothing new for many ground vehicles.
Fourthly, in this case no armor does equal best armor. If you reverse out facing the direction you want to shoot, you can lose all three crew members on the right side of the turret without your gunner being effected and there’s no ammo there, so you can bait enemies into shooting it. Keep in mind if you take more than 20 rounds of ammo, there will be ammo stored at the bottom of the hull, which when penetrated some spall can go into that and set it off, just a warning.
Fifthly, despite having no rangefinder it’s quite easy to get the hang of how the ammo handles. Fairly high velocity HE (680 m/s iirc) means most stuff you barely need to aim up for unless you’re going for a precise shot.
TL;DR, test drive it for a bit, hour or two if you really want to. Take some time getting used to how it handles, how the gun handles, what you can kill and where you can and can’t kill those targets at.
Hope this helps you somewhat o7