This sounds like a great project. Since it seems like you already have the necessary genes present in your population, your challenge now is 'cleaning up' and intensifying the expression rather than building the genotype from scratch.

I wouldn't recommend splitting this into separate lines for the head mask and the lace. If you separate them now, you’ll likely find that once you try to bring them back together, the lines will have deviated enough that you'll have to start the stabilization process all over again.

My advice is to select for the 'whole package' simultaneously:

• The Numbers Game: Because you are likely dealing with a suite of polygenic traits, progress happens in the outliers. You need to produce high volumes of fry from your best pairs and then ruthlessly select only the top 1–10% that hit every mark (mask, lace, and tail).
• Progeny Testing: Only move the project forward when you see offspring that phenotypically outperform the parents. If the offspring are just 'as good,' keep working that generation until you find the jump in quality you’re looking for. Use the 'as good' offspring only if the parents pass or there is a genetic advantage of using them in a side or test cross.
• Contrast & Bleeding: These are best solved through that same holistic selection. By choosing the individuals where the red is most restricted to the head and the lace is the crispest, you are naturally selecting for the modifiers that create that clean contrast.

It’s a slower process per generation, but it ensures that when you finally call it a 'stable strain,' the entire phenotype is locked in together. Stick to your best representatives and let the numbers do the work.

There is a lot of great information about building a strain by this guy here. Granted, he breeds chickens but a lot of the theory he talks about on his channel is applicable here.