That’s some sweet eyepieces you are starting off with, you will enjoy those a lot.
I have lots of 2” filter options for mono viewing, and it’s a one second job to slide them in and out of view in the filter slide. I find it a bit of an effort to swap multiple filters and eyepieces each time in the binoscope. So I bought the most versatile filter In the Lumicon UHC. I bought one pair of those in 1.25” and that satisfies all my filter desires in the binoscopes.
They are very enjoyable in the APM120, especially panning up and down the Milky Way at low and mid power. That’s from my dark site, but they will be valuable at Bortle 5 as well.
I own no ND or polarising type filters, and I think they are especially unnecessary in the APM120.
So my tip is start with a pair of narrow band, UHC type filters of your choice. You may find like me that’s all you need. I’ve used all the top brands and rate the Lumicon and TeleVue as best. The Astronomik and DGM NPB are very good as well.