Summarizing items duration into a timeline

Hello,
I’d like to discover a way to calculate a timeline for each of our projects rather than having to fill (and later update) them manually.
In our board each item is a project, whereas subitems are separate tasks which the R&D team defines and estimates their duration in man-days. I started by creating a linked field in the project which sums up the duration of all its subitems:


However, the resulting field type isn’t a number according to Monday’s strong-type rules, so the timeline field refuses to take it to calculate its duration:

Does anyone knows of a way of accomplishing this? I saw a couple of paid automation options which I’m considering, but even with those I’ll have to go and calculate all the existing items in my board.
Any help will be much appreciated!