I might have a few suggestions for the castle interior bgm that might fit perfectly, here's a few that you might consider, and I HIGHLY RECOMMEND that you leave the non-special/boss-type battle bgm off so that the castle interior bgm continuously plays as each one can fit PERFECTLY for battles as well (as in, it's a "Background Music Override" trope as described here: https://tvtropes.org/pmwiki/pmwiki.php/Main/BackgroundMusicOverride ), no bullshit. ^^ (I had to upload them on SoundCloud as they aren't on Youtube or any easily-found source otherwise it seems, though I made absolutely sure to list it in the description that I both did NOT make them and that they were from the source I extracted them from) https://soundcloud.com/user-954439293/sidereal-eye https://soundcloud.com/user-954439293/halderos-01
I was already thinking about having no battle music while in the castle, as there are a few tracks I’ve found that I want to include and having the standard battle theme interrupting it every so often would take away from those songs.
I also really like the song suggestions you’ve made. I’m going to stick to music that is publicly available for use in personal projects. Do you know if these tracks are royalty free? Can you link to the song’s original author so that I can properly credit them if I decide to use them?
Thank you for the suggestions.
I believe that they are fine to use since the RPGMaker title labeled "Theia - The Crimson Eclipse" uses them and I haven't heard of them getting into any legal trouble. Hell, it's been out for years and no legal issue has arrived at them yet I believe. It even uses a song from Chris Redfield's first campaign mission from RE6 (as I recognized it immediately when it played, having gone through RE6 a few times over on my 360/PS3 and all. This is the theme in case you were wondering:
though it cuts off at around 1:28 for the Theia variation of the song used) no less and Theia still has it in its current version so needless to say they aren't in any legal trouble, though I can understand your hesitation. Although they DO show a note at the startup of Theia to let any and all know that all original material used in Theia belongs to their respective original creators and in no way belong to them as detailed in this screenshot: 
Unfortunately, I don't know the original authors of the two songs I showed you (I was originally going to include this one here as another suggestion: https://soundcloud.com/user-954439293/dungeon-9 but then I thought it didn't infer enough of a "dread type of feeling" like the other two do and instead it feels like a more "courageous" feeling instead, so yeah... ^^; ), though as long as you detail that those songs weren't created by you and give out a note similar to the one shown in the screenshot then it should be alright.