I've hit 10K today. Most of my activity during that time has been trivial.
Congratulations! And don't knock yourself: I've been aware of you for a long time due to some of your nice answers in the Tolkien tags - not trivial at all.
Is it possible to relinquish the moderator tools?
No, there is no way to do this. Apart from, of course, simply not using your new powers. Or, as Adamant suggests, giving away bounties to reduce your rep - but then this is going to be an ongoing 'problem' as you'll keep getting more rep from drive-by upvotes.
The reason I'm uncomfortable is in case I ever lose control of this account. I dislike having unnecessary permissions on an account.
Don't worry too much about it. The term "moderator tools" is a bit of a misnomer. They're tools to assist in site moderation, yes, but so are the close-vote privilege at 3k and the edit privilege at 2k. They don't give you all the powers of what's usually called "moderators" on SE. You mention that the reason you're worried is the possibility of losing control of your account in the future, but relax because there's very little a single 10k-rep user can do to sabotage the site.
- Most of the 10k privileges are purely informational: you can see deleted posts, see the full history of the review queues, see lots of useful statistics in the so-called "moderator tools" page. None of that gives you any extra powers which could be used for evil.
- The main thing you can actually do which you couldn't before is vote to delete questions. But note "vote to". Just like with closing, deleting questions needs several high-rep users (in this case, at least three) to actually complete the deletion. Also, you can only vote to delete questions which have been closed for at least two days, and most such questions technically deserve deletion anyway (although on SFF we're not too fussy about actually going round deleting them all).
- If a troll took over your account, the worst things they could do would be things that don't involve the 10k privileges at all: posting nonsense, for example, or deleting your high-quality answers. These things could still be done even if you bounty away all your rep first.
- Finally, there are checks and balances. If weird behaviour started coming from your account, a moderator could "clear sessions", automatically logging you out of SE on all devices - that handles the case where a troll got hold of your computer without knowing your password. If it continued after that, we could temporarily suspend your account and reach out to you to find out what was going on.
So all in all, you're just another
brick in the wall 10k user with a lot of new visibility on the site but no new unilateral powers. Nobody can destroy the site with that account unless they gain access to a few others too.