If you like to have most of the simplicity of just setting classpath, but still have convenient dependency resolution, I highly recommend checkig out coursier: https://get-coursier.io/

Its a cli based artifact fetching tool for the JVM ecosystem, and you can have it just make up classpath strings for you, i.e., to use some Apache dependencies with the single file simplified java running you could just:

java --enable-preview --class-path (cs fetch --classpath org.apache.commons:commons-collections4:4.4) program.java

Also, maybe do have a look at scala-cli: https://scala-cli.virtuslab.org/ It does work just fine with Java files, and allows you to write single-file programs that include dependencies as special comments. The tool has some idiosyncrasies (like a background compiler server), but overall works extremely well if you need some of the power of a build tool, but without having a complex project structure.