Weeeell, in my case I usually tend to visualize my story as an anime, thus I imagine my characters having japanese voice actors lol. Alternatively, I imagined them being played by italian voice actors, since I'm italian sooo... yeah I still don't have names for a possible english dub.
Anyways, I'll go with the japanese VAs because you might be more familiar with them:
Frederick - Kensho Ono (Giorno Giovanna from Jojo's/Floch from Attack on Titan)
Nigel - Daiki Yamashita (Deku from MHA)
Kay - Yuichi Nakamura (Bruno Bucciarati from Jojo's/Satoru Gojo from Jujutsu Kaisen)
Drake - Yoshimasa Hosoya (Reiner Braun from Attack on Titan)
The Shadow Horseman/Dark Lord - Katsuyuki Konishi (Tengen Uzui from Demon Slayer)
I also thought of making a video where I might show my headcanon voices the same way you did, but to show a small example I do have two animations I made with the japanese voices. Here you go!
Frederick's voice https://www.instagram.com/p/CY7NEyFo1xx/?utm_source=ig_web_copy_link&igshid=MzRlODBiNWFlZA==
Nigel's voice https://www.instagram.com/p/CYUqx-5o8ST/?utm_source=ig_web_copy_link&igshid=MzRlODBiNWFlZA==