I would like to reduce my font size but keep the text centered in its current position. When I reduce the font size the text stays aligned to the left and bottom of its previous position. What is the easiest way to achieve this?

Does it work if you center the text before you change the size? It's only going to center horizontally, but that's better than nothing, I guess.

Or maybe it's possible with matrices in the transform dialog (but please don't ask me how... no idea about the numbers one would need to insert to scale around an object's center...).

(Btw. are these potato genes ? )

I'd try using the transforms dialog... use scaling by percent with proportional selected and separate objects selected.

Easy to select a text object and use select-same to get all the text selected.

Ha, right. The transform dialog scales around the center by default (and doesn't offer an option to change that). I didn't know, I almost never use it.