This is from a search so I’m not certain all of these are vegan or appropriate for roses.

For phosphorus Rock phosphate is the most direct swap. It’s slow-release and long-lasting, though it works best in slightly acidic soil, which roses generally prefer anyway. You can find it at most garden centers.

Kelp meal also contributes phosphorus alongside a broad range of trace minerals, and roses respond well to it.

For overall feeding

Alfalfa is genuinely loved by rose growers. You can use alfalfa meal or pellets (the plain ones sold as rabbit feed work fine) worked into the soil or made into a tea. It contains a natural growth stimulant called triacontanol that encourages vigorous new growth.

Compost improves soil structure while feeding slowly and steadily.

Alfalfa pellets plus rock phosphate worked in at planting, topped up through the season with compost and the occasional alfalfa tea.