Larian has access to your game saves, and so if they wanted they could deduce your play style and so parts of your personality from it, things which are useful for targeted ads and other things.
If you say Larian can be trusted, this is probably not much of a concern, I mention this only because of Twitch. Twitch relies on data mining, and if Larian analyzed your saves for things like above, they they would have things to trade. But I have no idea about BD3, maybe they know more significant things than that.
With Twitch, it’s a little different.
First, they obviously (because they have personal recommendations) collect information on your interests, like what content do you prefer, but as you said this does not apply to you.
However Twitch also has several mechanisms for getting you to give up your phone number or email address: they don’t support standard time based 2FA authentication, instead they can only send codes in SMS or email, and 2FA is required if you want to stream (even only once) or get your own API key to be use for archiving programs and such using the twitch API.
The problem is not actually connecting Twitch with your Larian account, it is connecting it with anything at all, and if the other connected account also does data mining then it’s even worse, because if the collected information gets to Twitch, they will probably be able to tie it to a real world identity (like a phone number, payment account, or a honest primary email address (that may even include your real name)), but also some history on how easy is it to get you to spend money, or just how easy it is to get your attention, and then pass it all off as something even more valuable to “business partners”.
Also, I’m not actually sure if Twitch would bother with support for connecting accounts of services from which they can’t extract much value.
So my main point is that I don’t know about Larian, but Twitch is only in it to gain something from it, and they would scoop up whatever Larian makes available.
OP is concerned about their privacy, rightfully so. Them requiring you to make an online account after you have purchased the game is entirely unjustified. They could already tie information to the account with which you purchased a licence.
Connecting Twitch and whatnot would work against your privacy: even if Larian does not do anything bad, they will share data with these connected accounts.
First, you won’t spend that money, you can spend that on other things instead.
Second, you can spend the money you have saved this way on products of better companies. For games this may be good indie developers and smaller studios (is that a thing?), but generally for software there is usually a wider range of options, and I mean even actual alternatives.
You could argue that me not paying for youtube premium won’t change a thing. That may sound true, but it isn’t necessarily: if you instead support your creators trough Liberapay or Patreon, then not only Google will get less, but the crearor and toss other platform will get more money, so they can improve their services and keep the lights up. Or like choosing to pay for Cryptpad instead of Google Drive will again besides having Google and their investors getting less, Cryptpad devs (who are very resource constrained, just as mostly any users-first software project because of not being known) will get more.