Using Project to plan a resource's work completion dates

Good morning,

I've got a single person working on different tasks.

The main task is a "background task", representing a set number of working hours every month (the actual amount of work may vary from month to month).

In parallel, there are smaller side tasks, with their own given amount of work.

What I'm trying to do is to set up the background task (fixed duration) one month at the time so I can manually adjust the work planned on a given month.

Then, I'd like Project to automatically place the side tasks so my resource is working at 100% all the time and I get an estimation of the finish date for said side tasks.

I already set up the resource calendar to fit reality (36h/week).

It's not working as intended despite my resource working only at ~24% on the background task: when I right-click "reschedule to available date", they always move far in future instead of being concurrent with the "background task".

Is there a way to resolve this please?

NB: I can't manually set the %unit for my ressource on each task, as I have no idea how much work will be allocated to the monthly task and this amount may fluctuate a lot.