It seems to me that you need to buy a complete cornie setup.
Any of the other options you are considering will require you to have a complete cornie setup. Growlers are quite expensive. You are surely not considering buying sufficient growlers to take a complete brew. You will be filling a few growlers from ...... A cornie setup.
If you decide to go for mini kegs, you could buy sufficient to take a complete brew, but what about a second or third brew on the go at the same time? Do you buy another 8 or 10? And then, as you've added sugar to the keg for a secondary fermentation, you have to be careful about trying not to disturb the sediment! Otherwise, you could fill the mini kegs directly from ........ A cornie setup. You could use these from the fridge with a Party Star tap, or with the likes of a Klarstein.
Whichever way you approach it, a full cornie setup seems somewhat inevitable. Either that or save your money and stick to bottles!
'9 litres jobbies' are almost impossible to find used, and new are as expensive, or more expensive than the full size cornies. It makes sense to put a brew into one full size cornie and a few bottles, or one full size cornie and a 5l minikeg, and then transferring to a 9l keg if and when needed. This means you could probably get by with one or two 9l kegs, and the rest being full size.
My point is that if you are going to move away from bottles, you will almost certainly require a full size cornie setup, whether or not you branch off into the other options. The level of investment is quite considerable, but it is not really possible to succeed with a half-hearted attempt.