Here are your options:
1) Post clips from your Youtube channel as they are know that you will be using an “original sound” if you do it that way and that you may not get the traffic you would get if you used a trending sound but that you would still feed over to the FYP however Tik Tok does value original sounds as well and its possible other people could use just the audio from your clip
2) Add a trending or copyrighted song to your tik tok clips and know that you won’t be penalised for that on Tik Tok like you would on Youtube
3) You can add a trending sound and set the volume to very low. That way you would gain exposure from the song but your content wouldn’t be changed
It sounds like you could benefit from spending a bit more time on Tik Tok as an app. People think its easy views and exposure but the people who do the best on there actually understand the platform well. In terms of gaining views on your Youtube, if you don’t already you should be using Tubebuddy which will give you an SEO checklist when you upload each video giving you the best chance of gaining views