Hmm when you're learning a new bator - stagger hatching does make things really a lot harder.
For one thing you don't know if it's the bator or the result of staggering that hurts your hatch rate.
What you pay for a bator has little to do with whether it works well or not. I paid probably less than 30 for both the homemades I have running now and they work great.
For staggering, keep your time turning eggs short, and fast.
I can't help you with humidity, it varies too much region to region and even house to house. Here I dry hatch and usually only tinker with the humidity if it's below 45% in the last three days.
Today I got caught with my pants down - evidentally a hen had started a couple of these for a few days before the eggs were found and gathered. I went to turn and found two pipping, when they should have been due th/fri/sat. Chuckle. Now they're dry, the incubator was running around 25%. Surprises...
Turn quickly and often, I do it 3-5 times a day. Trying to keep humidity and temps as stable as possible.
Twice works, mostly, some chicks don't do well that way, depending on the source of the eggs. Some do better. welcome to no rule set in concrete.
I use cartons to keep turning safe and efficient. It's a lot easier to bump or crack individually handled eggs.
If I'm staggering I put the hatching group on it's sides, the cartons then keep the others safe from bumper chicks. It also helps me remember who not to turn - I need all the help I can get LOL.
Mostly it takes practice. Next time try one source for eggs and one setting at a time.
But your major worries are consistent temps, and enough ventilation, and a bit of humidity at hatch.
Humidity you get to learn the hard way. I tried the normal way and everything drowned.
I forgot to raise it for one hatch, and everything hatched... clue phone...
So you have to play it by ear. The biggest cautions are enough ventilation, stable temps and remember every time you touch an egg you increase the chances you are going to crack or break one - broken eggs make unhappy people. Some small types of cracks can be mended with nail polish or super glue or candle wax dripped on, and the chick may go on to hatch.
Excess candling can result in lost chicks, so the "over candling" that I do for certain batches I do, knowing the risk. Those are never with vital eggs.