In the very long worlds, plane choice is easy, because your only 2 options are DC6 & Connie.
Micromanaging prices - I can't be bothered.
Leasing cheaper planes - yeah, maybe, if supply on the UM is low. You should lease them only as long as you expect to be flying them for. Never lease anything past a D-check. The saving from say an 18 month lease vs a 4.5 year lease isn't really that much, but if you know you'll use it for 4.5 years and then return it to be D-checked, or if you're right on 0 cash, then nothing wrong with taking the slightly longer lease. Never lease a new plane for anything other than 5 years though.
RI on a lot of routes - yeah, usually. May slow you down very slightly in the immediate-term, but probably pays off in the short-medium term. Would certainly want to get RI started on all of my larger, shorter routes.
There isn't one way to do it. But as a rule of thumb, you're likely going to get 5 planes in your initial start, you're going to need to pay for slots for all 5. If the planes are expensive, or slots are particularly expensive, or initial cash is relatively low, then maybe you want to fly longer routes and so buy less slots. If you have huge routes that will fly full from day 1, e.g. LHR-JFK, or Sydney-Melbourne, or Japanese domestic stuff, you're going to want to jump on those immediately. If you have 5 planes making immediate profit, that snowballs, you can afford more, and the limiting factor becomes acquiring them from the UM. As the UM starts to dry up, you can't keep leasaing stuff, more cash will accumulate, you can start looking at buying a used plane or two, to then have a secured loan and look at a new plane order.
For example, if I have a route with 300 supply and 1000 demand, and another with 0 supply and 500 demand, which route is better to open? Is absolute demand more valuable than the ratio of open demand? How does open demand, or the ratio of open demand effect load factors on zero image routes?
Don't worry about competition, especially on startup. Assume that you're going to get maybe 10% of demand at 0 RI. And your competitor will get a different 10% with their 0 RI. So the competition is basically irrelevant, you'll get 50 pax on your first flight on that 500 demand route, regardless of whether you have it to yourself or 6 other airlines are also flying it. The other consideration there is slot costs, particularly destination slot costs. A 300 demand route to a smaller airport might be better than a 500 demand route to a bigger one, purely because it saves cash upfront.
If you are trying to get off to a very fast start, the only real constants are that you're going to be quite aggressive with cash management, hovering around 0 cash or even into the negatives, because all available cash is used to increase growth, at least until you run out of easily available planes. That might mean buying slots only as planes are delivered, certainly means assinging routes to planes and therefore needing to hire staff at the last possible day, or taking cash down to 0 before the 12pm expenses come out, and then being in negative cash after midday. And that you'll be wanting to get 3 planes/week from the UM as much as possible. you'll almost certainly want to be doing it with a large fleet, medium planes typically cost you too much in slots, very large typically cost too much in upfront lease costs. and depending on the UM, you might end up using a temporary startup fleet just because of availability.
However, now I really want to try a world where I smart from the beginning, and try establish a near monopoly in a large base.
That is dependent on others in your base either giving up, getting bored, making a big error, or being poorly run in general. doesn't matter how fast your start, how good you are, if luck of the draw gives you 2 very good players in your HQ, and they don't make any major errors, then a near monopoly is simply not possible. If the luck of the draw gives you 6 other airlines that are all poorly run, not very ambitious, prone to errors, etc, then you can eventually get a near monopoly, regardless of whether you've gotten off to a very fast start or not.