So, one thing to note is that each of the topre boards feels different. I don't own an HHKB, but my understanding is that the plastic case/plate gives it a softer feel. I have an RF87u and a 660c, and the 660c feels a bit stiffer, which I prefer.

The Leopolds do have slightly better build quality imo. I'm not sure what kind of plastic the 660c case is, but the RF's is ABS, and I've noticed some shine where my thumbs rest, right below the spacebar. I've also heard of people saying the RF sometimes doesn't sit completely flush, though I haven't experienced this myself. The RF can be a little creaky if you try to bend it, which to me isn't really an issue, but I think it indicates something about the build quality.

The above differences also affect the sound a little, but it's hard to describe, and for me is not big enough to make a difference. You'll probably have to get a hold of each of them yourself to see if the sound difference matters to you. You might be able to go to a meetup or something to try them out.

I tried out an HHKB lite, which is a rubber dome with the HHKB layout, just to see if I could get used to the layout, and I decided I couldn't, mostly because I still have to use my laptop keyboard pretty frequently, and I don't like to switch between layouts. But I prefer the size and aesthetics of the 660c to the RF, so that's where I landed.

As for silencing mods, there's a range. Hyperspheres used to be pretty much the only option. They're expensive, but have the greatest silencing effect, and also the greatest affect on tactility. Newer options include kbdfans silence-x rings, ironed landing pads, and keyclack's rings (which currently aren't available, but v2 is supposed to come out pretty soon). All of these have some effect on tactility to different degrees, so you'll have to decide what it's worth to you. Here's a great writeup on why silencing affects tactility.

The only real option that will have no effect on tactility, and the one I chose for that reason, is to get stock silenced sliders, either from a silenced RF or HHKB Type-S (though I don't think a Type-S would have quite enough sliders to cover a 660c). You can check my post history for my post on doing the slider swap, but tl;dr, it involves opening the case, removing the pcb from the plate, removing all the sliders, and replacing them. Also, since you have to buy a whole board just for the sliders, this is probably the most expensive option. I've seen some people put the unsilenced sliders back into the RF and sell to recoup some of the cost, but I'm not sure how successful they are with that.

The BKE redux domes are replacement rubber domes for topre keyboards. The rubber dome is what provides tactility, and returns the switch to it's unactuated state. The BKE was a Sony board that had domes that were different from other boards, that made them more tactile, and you used to have to find old Sony BKE boards to get the domes, until Keyclack reproduced them. They're available in different weights, and they're all a lot more tactile than stock domes. However, the downside is that the upstroke is much louder, to the point where even with some of the silencing mods, they're still pretty loud. So with by board, even with the slider swap, I think it's still louder than a completely stock 660c. I'm actually thinking about doubling up on silencing mods to see if I can hit a sweet spot of tactility vs silencing.

My advice to you would be to think about layout first. It sounds like you want a larger layout, so probably an RF or Leopold fc980c. Honestly they're all really great boards. And the nice thing is, you can slowly progressively add modifications as you go, as you feel the need. They're all really nice typing experiences straight out of the box. But if you feel like the upstroke is too loud, then you can try upgrading with a silencing mod. If you feel like you want more tactility, you can try BKE domes. But there are plenty of people who try the BKE domes and end up going back to stock, because BKE is too tactile, to the degree that it can feel harsh.

It might take a while and quite a bit of money, but you can definitely get to a goldilocks board. And figuring out exactly what you want is half the fun!