price should be primarily affected by how much work you do. The whole point of a ych is to save yourself work, but there will still be a lot of variance depending on characters

I think your prices are fine even though I can rarely afford them.

Prices: I recommend charging 10%-20% more, or at a level you are comfortable with.
Taking on a commission needs to be worth the time spent on it in accordance to your budget/cost of living.

Multiple YCH: That’s a great idea to save time, the price can be fixed, and has potential for more users to snag a slot. Maybe offer a color sketch or a full render version, if someone wants to pay more or less.

Themes: Metal bondage and chains with NSFW content.
But I’d also like to see more expansion on magic in SFW art on the world of TC.

I think in terms of themes, stuff relating to the TC world or similar. That way it makes people feel more involved with your story. In terms of prices, I'm not sure what to suggest there.

If for any future TC ychs maybe situations or settings outside the usual golden collar and silken sash stuff?

deff interested in multislots. if that will be a thing.
Would also be a nice thing if they would feature the TCif world.

Pricing would be a more trickier thing for some furs, mainly because it's in GBP. which means in eur and usd it is more expensive, but in your country is more than a fair price.
So you may need to experiment a bit. if it goes a bit slow in finances, you can also do some headshots ych, those would be cheaper than full body's.

I dont think I'd do TC themes for YCHs since it's restrictive on what I allow character design wise, I'm somewhat strict about keeping TC stuff canon to the setting =P
TC-ify could be an option though for those interested, potentially with an extra fee if I have to do a lot of adapting with a character design to fit.