I haven't done a crossbreed project but I've been working on bringing all available genes in TB (including all the W genes, even though I'm not quite there yet)

Assuming you would get the W genes from foundation horses, I would say you should start with collecting the W genes and from there, decide your breed composition. You might figure preferences for your breed composition from collecting the genes ("this particular white looks stunning but can't be found in the other breeds so I'll need that in my composition").
After that, I personally would work on making a herd of mostly white horses of the crossbreed composition without care for PS/BS and without registering them. If you want to bring up PS early on, that means breeding your horses when they are older, therefore it would take a *lot* more time to have any progress on having mostly white horses. For BS, I think it can be worked on over time alongside PS.

In conclusion, I would do collect genes -> make breed composition -> breed horses with the correct composition -> work to get mostly white horses -> register a first gen in crossbreed campaign -> work on PS/BS with the next generations.

I agree, good PS comes with age and if you wait long enough you might not be able to get all the colors you want from a horse before it retires (I do this to myself constantly). BS can be improved at any time, once you have a good collection of horses with the genes you want you can start paying more attention to what pairings make good BS.

I would also add that you'll want to get your breed comp nailed down before you worry about BS. you'll need to know your breeds and average out their breed standards to figure out what the ideal standard is for your crossbreed.

From my own experience, I usually wait until horses are older to breed them to get good PS, and because of this I have been trying to breed a grade Marwari since we were first able to breed grades lol.

Thank you both, that's super helpful! I just couldn't wrap my head around which way round to go! I have a custom coin and I've been doing a lot of previews of W genes with that and I'm slowly looking for white horses of any breed to collect to try and get a variety of the white genes. (The TB W gene is a great one!)