Something that was pointed out to me in the comments by danjovic is that the video signal of the original ACE would probably need some additional conditioning. The signale misses something that is called 'backporch', which is essentially a short period following the sync pulse where the signal level is exacly 0V. The details are explained and shown in this nice video.

It's something that is also often mentioned regarding the ZX8/81 and Spectrum and therefore there are many circuit designs available that solve this. The one I liked most is the one which is also referred to in the video. It uses a NE555 chip, which seems sufficiently retro to me.

(The document where I took this from says it could be freely copied, as long as it contains the copyright message)

So I will add this to my schematic as well.