What are the best customizable routers nowadays?

Literally anything that's not ISP provided should give you the flexibility to set your own network parameters, but if you want strong flexibility beyond that, I'm going to throw my recommendation to opnSense, it's a fork of pfSense and the only reason I like it over pfSense is that the interface is very different and to me, it makes a lot more sense in how it's laid out.

That's personal preference, YMMV.

For consumer gear, ubiquiti has some strong units, the ER-X is pretty reasonable, but the unifi line is somewhat more beginner friendly, but tends to bury advanced features a little bit, focusing more on usage and reporting of activity and such.

Lower end consumer, the usual contenders are tp-link, and Netgear, though I lean more towards stuff from Asus, or anything on the dd-wrt compatibility lists... I ran a Linksys WRT54GL for a long time because of dd-wrt. I haven't kept up with the "wrt" variants over time.... The last time I touched dd-wrt was on a relatively high end (at the time) Asus router and it did very well... Might be work looking into. There's usually a trick to getting wrt firmware into a router though, and it will likely void your warranty, so buyer beware.

Circling back... My biggest issue with opnSense and pfSense, is the choice of hardware, unless you're buying direct from pfSense's netgate product line, you'll have to source something to run it on, and my biggest issue with that, personally, is that I want something small, like a router, IMO, should be, at least smaller than most PC's, that's relatively inexpensive, with at least two built in ethernet ports, since I've found that USB ethernet options are generally not very reliable. And usually, I can find something small and cheap, but there's only one ethernet port, or I can find something cheap with plenty of networking, but it's not small, or I can find something small with plenty of networking, but it's not cheap.

So I'm running a sonicwall at home, because fuck all this other junk, I just want something that does what I want it to do without hiding all my options behind some garbage, or a system that can only work a particular way, and you don't get options to change it. Or something that's huge or expensive.... Or something I have to spend a lot of time setting up, maintaining, or fixing. For me, that's sonicwall.