I'll just quote the OpenWRT Wiki here, because I think half the comments here confuse mesh and roaming:

Are you sure you want a mesh?

If you are looking for a solution to enable your user devices to seamlessly roam from one access point to another in your home, you need 802.11r (roaming), not 802.11s.

It is unfortunate that some manufacturers have used the word “Mesh” for marketing purposes to describe their non-standard, closed source, proprietary “roaming” functionality and this causes great confusion to many people when they enter the world of international standards and open source firmware for their network infrastructure.

• The accepted standard for mesh networks is ieee802.11s.
• The accepted standard for fast roaming of user devices is ieee802.11r.

These are two completely unrelated standards.

Source: https://openwrt.org/docs/guide-user/network/wifi/mesh/802-11s#are_you_sure_you_want_a_mesh

Well, I'm cheap so instead of spending 300 bucks on a Google router I bought three Xiaomi ax3000T for €35 a piece. They are listed as compatible with openwrt but for the last couple of month I've just been using them with the stock firmware and the WiFi mesh with backhaul works great.

It's otherwise very basic though, it lacks many features you'd expect like guest AP and so on..

I have a full batman-adv mesh on openwrt routers. That's maybe what you are after? Full guide in the link https://cgomesu.com/blog/Mesh-networking-openwrt-batman/

What you're after, transparent wifi roaming, is actually mostly handled by the client; what you need is wifi access points that don't get in the way.

I don't have much experience with new OpenWRT supporting products, but the kicker is you only need one of them. If you have multiple routers, they will require some setup to play nice with each other. An "Access point" is just the wifi provider, can be hooked up to provide whatever the one router manages, and are generally cheaper than a router.

To that end, I'd suggest a single router, and multiple access points. I do this with Ubiquiti access points in my home, their PoE has been nice and they have been pretty "setup once and forget" for a few years now. I'm sure there are some other brands that'll do well; Ruckus and Mikrotik come to mind.

Just purchased the Orbi setup and the performance improvement over my gen one Google mesh was shocking. I expected an improvement but my Xbox went from ~40 mbs to 240 mbs for straight game download. I did keep my old Google mesh up and running also and only use it for phones/tablets.

NETGEAR Orbi Whole Home Tri-Band... https://www.amazon.com/dp/B08Z3FGSDH

I was able to get some Linksys E8450 routers for cheap (~$40 each) on eBay a couple years ago, but they are more expensive atm (but $70 each, which is cheaper than a lot of other options). They require a bit of work to get OpenWRT installed, but it's not bad once the work is done (I have received nearly 400 Mb/s when I am close to the wireless connection sometimes, which is what my internet is capped at). I have used them as a mesh with one as a main gateway and the rest as access points with additional Ethernet ports. They do have the OKD issue fixed now, too, and they're also Wi-Fi 6. I'm looking into getting a Banana Pi BPI-R3 or R4, though I am not sure how well they are with mesh specifically, but have seen a few forum threads of people having some issues with mesh on these specific routers. I hope to switch to fast roaming (wired backhaul) eventually.

~~If you're not looking to segregate your network or any other "advanced" network feature, you can't really get better value that Tplink Deco.~~

Edit: I misunderstood what the op was asking for. This answer is not valid for that.

I use Asus routers running the merlin (soft) fork of their Asuswrt firmware. There are a couple of binary closed source blobs, but the rest is open.

There are two models I've used for this over the years, the Linksys EA8300 and the WRT 1900AC. Here's how I did it both times (though I only got around to writing up my notes the second time.

I'm a huge fan of Ubiquiti APs, and run their Unifi controller on a Raspberry Pi. Sadly, their code is proprietary - but it basically just works.

"mesh" is a buzzword that doesn't make much sense (to me at least) if we are talking about wired and routers... what do you mean by it? can you describe your setup?

edit:

Let me clarify :)

Unless I'm mistaken, mesh means that one a bunch of devices, usually wireless access points, connected with each other (in a mesh) with possibly low-quality connections that automatically switch traffic for each other.

If you have ethernet running from the router to the APs, you always want to use that and so you don't want a mesh at all.