Highly recommend a structured bonus plan. Have clear KPIs for achieving bonus. Ensure you can measure KPIs easily, they roll-up to your objectives, and make everything transparent. These should not be discretionary, they are mechanical based on the plan presented.
After seeing what you "must" bonus based on the plan and results... then you can move to discretionary. But a structured plan is not a pre-requisite to discretionary bonuses.
Discretionary bonuses are those you give out as a windfall. Make a list of who is providing the most value to the operation and your expected outcome of giving a bonus (staff retention, make up for low wages, thank you for extra work, etc). Then start allocating your bonus budget based on the long-term impact to the business.
just my 2 cents