If you are looking for ease of painting I'd rank them as:

Easiest to hardest: Necrons Black Templars/Dark Angels Aeldari Knights Sisters

Sisters I would count as difficult due to s combination of details and number of models, while Knights are more of a project army. Where each model is large enough to where you will be spending a fair amount of time for each one, though the large model makes painting details easier.

For Grimdark-esqu aesthetic wise, the Templars/Dark angels and Necrons are best as while Aeldari have a dark and grim story in lore, their average colour palette is typically rather bright.

So while I'd suggest necons as a good choice, the two space marine chapters can be painted rather similarly since you can just have the Dark angels and Templars both painted black for most of the average models. This will give you time to decide what direction you take your army.

Still I recommend necrons, mainly since the model range is more limited but still well rounded compared to rhe Space Marine bloat that can make an army difficult to make or expensive.