The ballast is of a very simple and dumb self oscillating design (much like ones in CFL), it's got no preheat (instant start). There's a capacitive potential divider to get the midpoint voltage and the two transistors switching from midpoint to ground and positive to midpoint across the tube. And looking at it the LC circuit is solely composed of the choke, the feedback transformer (negligible) and the capacitor in series with the filaments.

Yep, these work good with the integrated starer lamps. The only condition the lamp should not be lacking the capacitor parallel to the starter, but it does not matter what exact capacitance is there, it tends to work with anything in the 1..10nF ballpark. And that is the range most capacitors on starters use to be...

I see, I looked at a few schematics and understood the small cap in series is just for kickstarting before oscillating (essentially charged with DC), after that because of the high frequency it would act like a dead short, then another larger capacitor (tens of uf) on the positive side of the lamp will make the LC resonate at a lower frequency, sort of acting as a capacitive dropper. So when combining the capacitive reactance of the cap and the inductive reactance of the choke, it seems like the current is regulated this way. Am I correct? Probably asking too much at this point, really thanks for your help.

I'm sorry, I hot lost which capacitor you mentioned is which. You may try torefer them e.g. according to this example

The C3 is the DC blocking capacitor, passing the operating HF current (very low impedance) but isolating the DC component. Often it is split into two units, one connected to positive and one to negative rail, it allows to separate the electrolytic (C1) by a small inductor, so relieve its current ripple loading.
The L1 is the main ballasting coil, limiting the lamp arc operating current (along with the operating frequency and the arc impedance).
The capacitor in the lamp starter assembly is effectively parallel to the arc, but in series with the filaments. When the rac is not existent yet, it forms a series LC with the L1 and "pulls" the operating frequency towards the corresponding resonant frequency, so it allows large current to flow, at the same time forms large voltage across the capacitor, so make the lamp to ignite.
And the capacitor in the lamp also completes the feedback circuit (signal picked up by the series connected feedback ring core transformer) when the lamp is not lit, so allows the oscillator to start.
After ignition, it becomes insignificant (has way higher reactance compare to the arc impedance), the ballast would operate the same way even when disconnected during operation, the current flows through the arc.

Cheers mate, now I've got it through. So the LC circuit is there to solely start the lamp with increased current, then it is gone and the frequency is only maintained by the saturation of the feedback transformer. So to half the output of this ballast I need two L1 in series, which there are two on board since it's designed for  2 tubes. Thanks for your help and knowledge!

To reduce the current you may need to alter the feedback transformer and the base resistors, as just by increasing inductance the circuit will compensate by reducing the frequency (if the feedback saturation current is the strongest factor there).
What the ballast you have was actually designed for?

The ballast is designed for 36W T8 tubes, gate drive resistor is 15 ohms, on the feedback transformer primary is 1 turn, secondary are 3 turns. If I put 1 more turn on the primary, isn't the saturation current going to be halved which will kinda solve it?

EDIT: did just that, shorted the 2 inductors and put an extra turn on the primary, works perfectly now, the starter cause the lamp to flash occasionally during startup but the lamp works very well.

What do you mean by "shorted the 2 inductors"?

"It works": Check the total power, it should not exceed the lamp power rating by more than 1..2W (assume roughly 10% losses in the ballast, but also the 10% reduction due to the steady capacitive current adding to the filament heat, to prevent filament overtemperature)...

Instant and sometimes few flashes starting is normal for such setup, that part works as expected.

Measured on the mains side it's about 0.1A (22-23 watts, my multimeter isn't very accurate). shorted would be the wrong word, I had a brain freeze, I meant putting them in series. The lamp may be slightly overdriven, maybe a watt or two.

OK, I see...
Yes, it seems reasonable...