I have a ROI goal and a fixed budget. What budget strategy should I be using? (I see separate for ROI and Daily budget)
The short answer to your question is that you can use either of the budget strategies (ROI or Daily) and not lose sight of your other objective.
If you use the Daily budget, you can continue to monitor your ROI by running reports and/or looking at Simulations. The Simulations will recommend a budget based on your ROI.
If you use the ROI strategy, you can set up a min and max daily budget within which you want to hit your ROI target.
If it helps, the most popular budget strategy is the Daily budget.