OH THIS THANK YOU @kerink!!! The simple fucking fact that Bill can literally rewire Ford's fucking brain to obey him if he wanted to (due to Ford AGREEING to possession) hits the nail on the head to display that Bill cares for who Ford is, and so vastly prefers Ford to join him 'willingly'. Afterall, if you just make someone love you (via fear, like Bill's usually ruled, or via brain rewiring...) it isn't the same as someone willingly choosing to love you, like Ford did, and Bill desperately wants that back (and subsequently leans on his usual tactics of fear to try and get it back once Ford refuses to speak to him). To digress, the actual extent of Bill's power is a bit unclear; especially as Bill lies, and the most powerful (beyond weirdmageddon) he tends to be is usually within the dreamscape emulated as real life, which he has immense control over... But then also you have direct evidence of Bill rewiring Ford brain with Fordtramarine, so that IS within his scope of powers, at least within the contract of possession. Also... taking a step back to a Dolyelist explanation, GF is a comedy and follows a narrative (especially looking at TBOB), so lots of Bill's powers directly stem from what makes sense narratively, and/or is something comedic, so I also have a distinct feeling Bill's powers less follow a Watsonian explanation.
Also I completely agree that Bill sees himself in Ford; they're so similar when they meet, and much like in some ways Ford projected onto Dipper, Bill projects himself onto Ford... and I think that's why he assumes Ford would be ultimately fine with Bill's take over, especially Ford openly does wish violent retribution regarding being treated poorly for being different (ie, Ford's comment about Rudolph). However (small peanuts here tbh) I wouldn't perhaps say he wanted the exact same experience from Ford, to destroy his dimension; I think already Ford's falling out with Stan is strong enough of a resemblance for that imprinting... Mmm it's more like I don't think it was a conscious thought within Bill, but also wouldn't be surprised if he would also be eager about that parallel if he was consciously aware of it (and may have been unconsciously eager about it). I think Bill really is a reactive person more so than someone who is actively planning what they are doing; I think that as a conscious desire in Bill is a bit too much for a character that is very reactive, and I think Bill more was driven by the desire of I want Ford by my side. And I think that also influenced his thoughts that Ford would end up agreeing with him; sometimes when people (especially if they're more self-focused) assume that their desires are their partners desires, (cough, projecting, like previously mentioned) even when it's very much not true.
I think (as a watsonian explanation) that's also why you have Bill ridiculing Ford when he confronts him about the portal; even though he wants Ford to continue to be at his side (and it doesn't make sense to ridicule him if it pushes Ford away). First of all, to a degree he DOES recognize what he's doing is bad even if he denies it 'as improving Earth', and that Ford has compunctions about what Bill's doing; that's the reason why he lies about it, but also mocks Ford in doing something he'd normally never agree to. Secondly, I don't think Bill at this point doesn't quite realize Ford means a LOT to him yet, and hasn't really thought much about the portal reveal, only just vague, it'll be fine, in the end he'll agree sentiment likely due to what I outlined above. Thirdly, Bill's self destructive and thrives on (unhealthy) power dynamics, especially as you know the two of them have had some strained interactions by this point, case in point when they both accuse each other of essentially cheating. It's Bill putting Ford into his place, and he can't resist that. And so it all culmulates into him doing what he ALWAYS does when the truth comes out, and he ridicules Ford. And it goes spectacularly sideways, for Bill especially as @itslikepoetrythat excellently points out, Bill immediately SPECIFICALLY ridicules Ford's intelligence, Ford's load-bearing concept of self-worth... on top of the fact that Bill has been providing Ford's sense of selfworth for a good long time now, especially with Bill's I'm a god of knowledge, and I choose you because you are so special and smart shill. Which of course has Ford feeling immensely betrayed by the person he's closest to, just like Stan, as everything around him that he's cared about and worked towards collapses all around him, but it's worse because he was played like a fiddle and on top of the horror that he's been building a portal to kickstart the end of the world. And so Ford, cuts Bill out of his life. Also like ehhhh more small peanuts perhaps in different opinions of Ford's internal structure in response to everything, but I would say it's less if I can't have this then you can't have this about the portal, but rather a reactive horror to the situation; that's not to say there isn't a revenge aspect at all, but I think it's a very reactive I want nothing to do with this (but also this is my life's work tension thrown in) and a very tangible desire to not want the end of the world to happen (who knows what Bill would do).
Anyhow. Damn these two are the gift that keeps on giving for character analysis...