How to water/fertilize trees with cacti around it?

I've been redoing my yard and adding a ton of cacti and succulents. Being in the Phoenix area, it limits the amount of things I can plant which can tolerate full sun so I've been using the trees to my advantage as best as I can. I have a palo verde and mesquite which are decent sized but extremely airy/wispy, offering very little shade. They do not grow all that much because I don't have them on irrigation since they became established. I'd like to fill in the canopy and get more height to create more partial shade, allowing me to plant a larger variety of unique cacti, euphorbia, and agave. The mesquite (not shown) has twice as many cacti around it

My question is, how do I go about watering and especially fertilizing the trees without creating issues for the plants around it? For fertilizing, I was going to use tree spikes but those have to be places every 3-4 feet in a circle that mirrors the edge of the tree canopy. I'm assuming the spikes will leach fertilizer to everything around them in the ground and I'm afraid I'm going to over-fertilize the rest of the plants. How do I go about watering the trees as well in a way that doesn't impact the cacti around it?