After a takeout with Google Photos any file taken with your own device should contain the correct time. But photos sent from other people, created with Lightroom or very old files might have a stripped date field. Google Photos usually adds a date from the date it was uploaded. You want to have at least that date from Google, otherwise all files you import to Synology Photos will clump together at the date of the indexation.
That's also what most people are doing but they think Google stripped the date from their files during the Takeout. Most files should be fine though, depending on your specific photo collection of course.
My current takeout size is 100GB and I didn't have that much space left while I have plenty of disk space on my synology.