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This is far from ideal because you won’t have as much control as you think since the ROAS is influenced by many different factors.
A more common approach is to set a consistent tROAS and run open budgets. Even with open budgets you should set a reasonable cap to avoid unexpected overruns… 25% is usually enough unless there is a lot of seasonality or activity during sales, etc… in which case you can go to 50%.
However, if you have budget caps you don’t need to run tROAS at all, switch over to Max Conversion Value with no target and let Google spend all of the budget while generating the best return possible.
Obviously as with any account you still need to monitor closely in case things go off the rails; in this case you might see ROAS tank while Google continues to spend fully.
If you want to invest more into certain products you should put those into a separate P-Max campaign. If you’re running standard shopping you can, of course run different tROAS figures for each ad group.
