I suspect your temps are all fine, your dose is consistent, but perhaps the simple lever action is being over thought a little.
I don't believe you should require excessive force to pull the shot, like you may see in some videos online. Pressure is needed but it needs to be a consistent, even pressure that gradually releases the shot, rather than firing it through. If you feel too much resistance, I usually find the dose is too high (not your case) or the grind is too small (could be your face).
When you move the lever up, with the portafilter locked in ready to go, just move it all the way to the top and wait 20 seconds. You can guarentee it'll be full and infused by then.
If coffee runs out immediately or after only a second or so, I found this meant the grind is too large or the dose too small or both.
Some say, and I think it may be preferred (?) that you should get a small run of coffee after 5-6 seconds, at which point you can pull the shot. BUT I don't believe this has to be the case, as you can work in your grinder to get the right amount of resistance in the pull and infusion. Mine is dialed in so it preinfused for 10-20 seconds but with no "drip" - but I can ease in as much water as I want, ultimately.
You will go through a few bags of coffee but always get good, fresh coffee. Don't be tempted to get some old crap from the supermarket, it'll just make your results more inconsistent and confusing!
Unless there is strong evidence / reason to believe the machine is faulty, I'd just keep working on the technique with fresh coffee for a good while first.