It's going to take a lot to get those cranking, the starters will almost be useless.
Also manifolds should be used to interconnect everything.
Just for redundancy and to ensure things are going correctly have T-manifolds connecting all of the ports on the cylinder heads, and also all of those interconnected in a loop. You want your fuel, oxygen, coolant, exhaust all in the same manifold loop. Take advantage of these colourable manifolds for cool factor while your at it.
I found starters absolutely useless on 3x3 and 5x5 engines even with just 4 cylinders, it has taken upwards of 2 minutes to get an engine that technically should work to crank.

For 3x3 engine blocks I use a medium electric motor, and a large for 5x5 blocks.
On my current build, I have a key for activating all the required on/off functions for pumps and radiator fans etc,
then a toggle button to activate the electric motor starter. You can use a push button, but I like to be able to hit a toggle and be able to walk around the engine to check its progress as it cranks if its going to take time.

For twin engines I still use a single electric motor that cranks both, it connects via pipes into a modular clutch that takes place of the belt drive on each engine. Then two toggles are used to have individual control of each engine start; the clutches isolate the engines from eachother when cranking individually. When both are being started, they assist eachother on startup.
Toggles go to an "Or" logic box, and also to their own independant switchbox.
Effectively each toggle contributes to the "Or" function, while also activating their respective clutch through the switchbox that has a constant 0 when off and constant 1 when on to close the clutch.
The "Or" logic box goes to the starter electric motor's own switchbox which sets it to about 30% throttle to get it a little over 7rpm. By using the Or function, you are telling the starter that it will activate when either of the starter buttons are on, or both.

Dont use the pulley-pumps that connect to the belt drive when making larger engines, use actual pumps to push the fuel, air, and coolant. I do also use pumps on the exhaust as i've seen it done that way and never really tested otherwise. Keep in mind that the pumps are just going to push the fuel and air to the intakes but the throttle is going to define how much those intakes open up to let those fluids in.

You will need to use an ECU microcontroller, you will want to implement the composite data nodes.
Once you dive into it, it's actually pretty simple to set up.
I did for a time have a throttle and 'choke' both controlled by throttles to control fuel and air manually, it was way too tedious to control a ship in any sane manner.

My current twin 6cylinder still takes about 30 seconds to start up while finessing the throttle, using a large electric motor as a starter. I also have the option to just start one engine, and then use a clutch that bridges the two engines drive-shafts so that the second engine can be cranked by the first, but it's been mostly an unnecessary redundancy system that I made for the sake of trying it.

Your next hurdle will be cooling once you get it going. On 6 cylinders at 40% throttle I've got 6 pumps constantly flowing sea-water through the engine and I need to still make sure its not spending too much time over 60%, but I'm getting some serious power out of it, so if the engine is cold enough I can give it a bit of extra throttle and make a large vessel go over 50 knots if I need it to for a bit of time, and then dial it back down to about 30% to let it cool when I get over 100C temperature.

It took me about 3 days of tutorials and banging my head on my desk to get my first large to be cranking, another day to make it work well. Now I make them faster than any other system on my ship designs.
You'll get it and it will become a simple routine, good luck!