In no particular order, here are some useful alternatives to JMeter, which all support "concurrent stress testing". They can each be used to script, schedule, execute, & report synthetic loads for APIs as well as for web servers.
- Gatling
- Locust
- Grinder
I do agree that JMeter is peculiar, but once you get used to it, it can get the job done, and it's got good community support. I particularly appreciate the wide range of plugins available.