My question is - what would you think is better humidity for the start - 23% or 40%? And if 23%, do you gradually increase the humidity up to 70/80% or just go for it??
For many different reasons different humidities work best for different people. Some people do well at lower humidities, others do better at relatively high humidities.
What matters is how much total moisture is lost during the incubation. If too much is lost or not enough then you are not likely to have a good hatch. Mother Nature was good to us in that she made a fairly wide range of humidities work, but there are limits.
The situation changes when you go into lockdown. The problem there is not how much total moisture you lose through the porous shell. The main problem is that if the humidity is too low when the chick external pips it can become shrink wrapped. The membrane around it dries out and shrinks to where the chick cannot move to hatch. Not every chick will become shrink wrapped if the humidity is low, again Mother Nature was pretty kind to us and gave a range, but the risk is high enough that the manufacturers warn us to raise humidity during lockdown.
Not all eggs hatch on exactly 21 days. Often they can hatch a full day or even two days early or late and do fine. That's why lockdown starts after 18 days of incubation, to be ready for the early hatchers. Again, there can be many different reasons for this. Mother Nature has helped us out here too. For example, before they hatch a healthy chick absorbs the yolk. It can live off of the nutrition and moisture in that yolk for over 72 hours in case it needs to wait on a late chick to hatch.
My suggestion is to determine a humidity and be as consistent as you can in maintaining that humidity. The manufacturers recommendations might be a good place to start. Keep records. It's average humidity that counts, not an instantaneous humidity. If your water reservoir runs dry the humidity can drop. As long as that is for a short time, no big deal. Sometimes the humidity may go up, either because you spilled water when filling your reservoir or maybe the background conditions changed. If something like that happens I may run it with low humidity to get to an average.
Evaluate your hatch. See how many hatch. Open unhatched eggs to see if you can determine why it did not hatch. If necessary tweak your humidity for the next hatch. This is what the professions that may hatch 1,000,000 chicks each week do when they get a new incubator or even move one to a new location in the same incubation room.
I'll link a couple of charts that may help you on your assessment.
Trouble Shooting Failures with Egg Incubation | Mississippi State University Extension Service (msstate.edu)
Common Incubation Problems: Causes and Remedies (ucanr.edu)
I guess my main concern is bacteria..... warm and wet seems really unnatural, and not at all like the dry, soft feathery brood patch I imagine under a mother hen....
Bacteria is a concern. If bacteria gets inside the egg incubation temperature is the perfect temperature for bacteria to grow. Scientists use raw egg as a great food to culture bactea. Before I start incubating I sterilize the incubator with a weak bleach solution. After incubation I clear it well to get rid of residue.
Only set clean eggs. I don't worry of it has a light dusting on it but do not set eggs that have clumps of dried poop or mud. Do not wash your eggs before incubation. One of the last things a hen does when she lays an egg is to put a wet coating of what we call bloom on it. That's why a just laid egg may look wet. The bloom quickly dries and is a great barrier to keep bacteria out while letting that porous shell breathe in fresh air for the developing chick. So don't wash the eggs or use sandpaper to remove clumps. Just don't set dirty eggs to start with.
A chicken can lay eggs in a hidden nest for two weeks and then hatch them without bacteria getting inside. Some poultry like geese, ducks, or turkeys can incubate for four or five weeks after laying a brood without bacteria getting inside, the bloom is that good.
For what it is worth, studies have shown that a broody hen can control the humidity under her.
and my eggs will arrive next week 
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