I think we need to know more details, but ASSUMING that we are talking about the standard 2-bulb interior lighting from Märklin, this is what would happen;
The lights were made for 16Vac, but will survive short 24Vac pulses.
Your digital voltage will probably be around 19V, so the light will be bright, and yes, it will probably flicker, and lifetime of the bulbs could drop, because of the higher voltage.
Being incandescent bulbs, they will also consume a lot of current. Around 1VA per bulb.
In the case of standard Märklin lighting kits, they won't damage your ECOS.

If it turns out that we are talking about a more modern, buffered, LED-equipped set, there should be no problems whatsoever!

I used them in the past and it was no problem on digital, but they do use a bit of the track power if you have a long train

are these the lights in the tin-plate cars. These are very bright and do use up a lot of power.
https://www.maerklin.de/...cts/details/article/7077

inside the coaches I have a few 7077 lights, in one I have a 484/3 light block.
I believe the pickup shoes are the 7198.
The restaurant cars, item 342/2, have the live or B wire coming from the roof pantograph, which I will remove using pickup shoes for both of them.
Correct bits identified?

Do you mean that you have isolated the 7077 lights completely from the metal-wagon, and then rectified the power coming from the slider and THEN added a capacitor?

(-Because in an AC environment, the capacitor will just be a resistance)

UPDATE: i placed the coach with the 50s light kit and the lights work, current used for three lights is 187 mA as shown on Ecos monitor.
I found similar sized LED bulbs on ebay and got them.
The set of 3 lights uses up 23 mA, which is good.
These are the LEDs on ebay: 325420780782
Problem, they flicker badly. Is there something I can do? Use a function decoder to power the lights? The seller proposed to send over the AC ones but I believe it will make no difference. Thank you