How to start multiple systemd services within a target sequentially

I have a large set of systemd services that are part of a single target running on an embedded system with a relatively slow file system. When starting the target, our most significant startup delay is due to loading the application from the filesystem into memory. When the target is started, SystemD starts all services simultaneously, and we occasionally miss the service's TimeoutSec delay for some applications. We could increase the TimeoutSec to increase the delay, but I'm interested in whether we can improve the overall system startup by having SystemD start only 1 service at a time and not thrashing the i/o. We could use the service unit wants/after/requires etc. to enforce a specific startup ordering, but the ordering doesn't matter. Is there a way, without requiring specific ordering, to tell SystemD to start the services within a target 1 at a time?