From the distance I would guess, that your older photos are in lack of the right data within the Exif-Part.
So I would have a look onto this, to see if you could correct this, before you upload them into Photos.
There are a lot of Apps out there to do so, and I haven’t used any of them during the last couple of years, so I could not make a serious recommendation for one, but on the following website, there are some of them with a description.
Once they are in Photos you can adjust the dates using Image → Adjust Date and Time. It’s tedious but it does work.
Another possibility is to use something like A Better Finder Attributes before importing. Ultimately I concluded this wasn’t significantly better than using the built-in tool, YMMV.