Sounds great! Good job ~
Here are some suggestions of mine for smoother video editors that are free to use:
Browser based editor with a lot of simple functions, has a watermark (albeit less intrusive) but also has a lot of useful assets that particularly work well with other software, like audio visualisers. (I personally use Veed for small assets, then chroma key a green screen to make them transparent and crop out the watermark in my actual editor)
I will preface by saying DaVinci Resolve is a demanding editor with a fair bit of a learning curve, especially if you’re new to video editors because it’s professional grade, it’s what filmmakers are using, BUT it’s completely free to use for the consumer grade version!
That’s the error kana that usually appears when the encoding of the oto file (the type of program text it uses) is wrong, so UTAU can’t read the kana and just sees gibberish.
https://notepad-plus-plus.org/downloads/
Open the oto in Notepad ++
Go to Encoding > Character Sets > Japanese > Shift JIS
That should hopefully fix it!
There're AI services like this that can separate the vocals for you. It may be possible to cut out the whole set of syllables required for a complete voice bank. There will be a lot of manual work involved to undo all the transformations (length, pitch) but the tools are readily available (eg Audacity)