Personally, I find that I output power slightly differently while climbing, while riding flats and while riding slight 1-2% downhill grades. So I try to do intervals in all three types of terrain. My recc is to focus your work on terrain you'll be riding in for whatever your event is. In your case there's no event, you just want to ride faster in your everyday terrain, which is flats. So I'd focus my interval work on flat terrain. Obviously a big descent will screw up an interval. But anything from -3% to +5-6% is great for most intervals IMO.

I'm predominantly a Time Trialist, so I work on outputting steady power in the range of grades I've listed above. The very interesting thing I've observed from riding with recreational friends is that I'm faster by virtue of the fact that I've learned how to stay on a number longer. Some of that is a fitness thing but some is mental, you're just more tenacious when you learn how to be tenacious. I learn a lot of that during very long intervals- sometimes 20, 40 or 60 minutes. Likely not appropriate intervals for you, so my point is not to advocate for them. It's more that learning to stick with a number for the entire interval I think can make you faster, regardless of FTP. It's a riding style thing.