would those containers best be started from something in services mode?

Yes as long as the machine has enough cpu/ram
Notice that the services mode will start a second parallel Task after the first one is done setting up the env, if running with CLEARML_AGENT_SKIP_PYTHON_ENV_INSTALL, with containers that have git/python/clearml-agent preinstalled it should be minimal.

or is it possible to get no-overhead with my approach of worker-inside-docker?

No do not do that, see above explanation on why CLEARML_AGENT_SKIP_PYTHON_ENV_INSTALL does not work in docker venv mode

i designed my tasks as different functions, based mostly on what metrics to report and artifacts that are best cached (and how to best leverage comparisons of tasks). they do require cpu, but not a ton.

just report a single Task as multiple "titles" then each title is it's own step, then inside the "title" they have different seriese

is there a way for me to toggle CLEARML's log level?

Try to set the python master logger base logging level