You've done so many things wrong.
Finding a good trading route is essential of course. Sure you need to find a trading route that provides you a profit of 1000Cr per ton bi-directional between systems at the distance of 1-2 jumps between each other. This should allow you to perform 3 bi-directional trading runs and results in about 120k Cr refuelling costs. Given that Python in cargo configuration with shields can carry up to 284t of cargo this results in 1.7M Cr profit before you need to refuel. So taking into account refuelling your profit is around 1.6M Cr. 1 bi-directional trading route usually requires 10-15 minutes, which means the profit of around 2M Cr per hour even if you buy fuel (at a very moderate pace).
If you do not want to buy fuel (or if you are not using a Python for trading) you equip it with a fuel scoop and this removes completely the necessity to refuel. When I was using a Python for trading I did not bother with a fuel scoop as it requires time to scoop fuel at a loss of cargo space.
If you know what to do then you will easily make enough Cr using a Python so no other ships are required. Python provides you the access to the outposts 50t larger cargo hold than a T7, faster velocity, better protection. Moreover, the chances that a Python is going to be interdicted are much lower than for a T7.
And what sense is to use a Python for trading if you cannot afford to trade carrying 272-284t of cargo?
If you are planning to buy another ship for trading it only makes sense to do it when your balance allows you to buy a new ship and trading more cargo than your current ship can carry.
That analysis fits squarely with my own experience.
I traded my T7 in for a python when I had around 74MCr in assets. (I am defensive trader in open BTW)
This got me a fully working ship with A4 shields, A5 FSD, class 6 reactor (all you need even with 3 C3 beams and a couple of multicannons) and 276 tons cargo capability (15LY jump at full load), with cash for cargo and re-buy.
I soon swapped out some cargo for an A3 fuel scoop, which at 176Kg/s refuels what I need in the time that I am lining up the next destination on my intermediate jump. For me it is more economic than 8T cargo.
With this ship, I could make 2.5MCr/h enabling fast upgrades.
It is now worth 105MCr, and can 'only' carry 252T cargo as I have gone to class 5 shields to avoid hull damage in combat (which is expensive to repair), plus some other upgrades.
I don't seem to need class 6 shields, and although I now trade in a ship where I don't worry about interdictions, I still make close to 2MCr/h. Plus it is a treat to fly - even post-nerf.
No shield cells! They seem like a cheat to me.