PMax asset groups: split by category or by margin band for a mixed catalog?

Scenario: a shop with about 400 SKUs, roughly 60% apparel at 35% margin and a chunk of electronics at 8%. The question is how to split the PMax asset groups. Option A is one asset group per category, so creative and audience signals match the product. Option B is grouping by margin band, so the ROAS logic can differ.

I lean toward category first. Asset groups mostly shape creative and signals, and tROAS is set at campaign level anyway. Margin would go into a custom label that drives a campaign split, but only where the gap is big enough to matter, and 35% against 8% clearly is.

What I'm not sure about is whether splitting campaigns just to hit different ROAS targets starves the low-volume group of learning. Has anyone run this on a catalog around this size, and what did you end up doing?

I'd push back a little on splitting campaigns just to set different ROAS targets. The split is exactly what starves the low-volume group: electronics at 8% margin may only convert a handful of times a month, and smart bidding needs volume to learn. A better route is to send margin-adjusted conversion values, so each sale counts as profit rather than revenue. Then one tROAS target works across both categories, and the algorithm weighs the apparel against the thinner electronics sales on its own. How many electronics conversions do you get in a typical month?

Margin-adjusted values are the right direction, but they change what your target means. If the purchase event sends profit instead of revenue, a 280% tROAS that works for apparel stops meaning anything. You'd have to recalculate the target as profit per ad dollar, and your reports will show profit as "conversion value" unless you keep revenue in a separate column.

The margin also has to come from somewhere. Google Ads conversion value rules can't read a product custom label, so the margin has to travel with the purchase event through your dataLayer or GTM. That's a tracking project, not a settings change.

Volume doesn't go away either. Value adjustment changes what each sale is worth to the bidder, not how many sales you get. If electronics only converts a handful of times a month, I'd still pull the top few electronics SKUs into their own campaign rather than splitting the whole catalog by category.