Linux

Honestly, I'd just use whatever the ISP provides.
Sure, it's not open source and it kinda sucks... But I mean, if you don't trust the ISP modem, you can't trust the rest of their infrastructure either anyway, so it's kinda moot.
At least that way you have a vague chance of having a modicum of support when shit breaks.

If it can't be put into bridge mode, it probably has some sort of DMZ function where it basically does port forwarding for any/all possible ports.
Double NAT isn't as bad as it sounds these days.

Now to your question...
They exist, they're mostly targeted at ISPs though, so might be a harder find than other things.
They might also be older, as basically all customers also want their ISP to provide Wi-Fi, which a bridge modem won't.
Anyway,
You'll have to know what DSL were talking about, there's... ADSL, ADSL2+, VDSL, VDSL2, etc

One old-ass model we used to use back in the day was.. a Siemens 5200, but that's ADSL2 at best, definitely not VDSL.

Not quite what you're seeking, but maybe relevant.

Have you considered an SFP modem like the Proscend 180T? Might be hard to source these days, but if you have a router with an SFP cage this would be a way to accomplish the single box goal.

Is there any reason you are not just calling your ISP and asking them to put their box into bridge mode and then setting your own router up behind it?

You control everything from the router and into your local network.

If you can't trust your ISPs "modem" in bridge mode, then you also can't trust the infrastructure beyond the modem, and in that case you would need to set up a privacy respecting VPN service directly on your router to encrypt the traffic your ISP sees.

Is this how one prevents viruses?? 😂

You don't say which modem you mean, but from context it seems to be xDSL. I'm running an own cable modem (Vodafone in Germany) which works a lot better than Vodafone's own POS (which insists to reboot itself every day or two despite being in bridge mode). There is a thin client running opnsense behind it.

As to picking the smallest and lowest power, I think it is better to pick one that works best. When I used DSL I bought a cheap hackable DSL modem from an italian ISP and put OpenWRT on it. It also ran in bridge mode. It had an SFP slot which can come handy for galvanic separation, but I didn't bother with it.

I would start with your ISP's list of supported devices.

I would use OpenWRT

Also, could you get fiber? A DSL connection in 2025 is going to be pretty slow.

If your dsl is G.Fast you could try the Zyxel GM4100. That device is only a bridge

Unfortunately modems have the issue that they are made to be provisioned, configured, and owned by the ISP. At least that's how most DOCSIS setups I've seen work.

The only hope I'm seeing (I'm also wanting to do this) is running my own CMTS. I found a service (https://github.com/cablelabs/os-provisioning is the only opensource tool I've seen before) but I am still trying to figure out a reasonable cost way to connect that server to the actual network (so far bladerf sdr is ny best hair brained idea, which is really only cost effective because I want to play with 5g too). The last bit of luck I have is that my ISP isn't actually doing firmware updates on my modems, so if I connected them to my Internet lab network, provisioned my custom OS and firmware, and then connected it back to the ISP I can get their DOCSIS config. In theory at least 😅

Netgear has modems that have their OS and firmware in nice packages with build instructions at least. ARIS's older modem code is out there too but a pain compared to get.

You technically could use that same sdr as the modem on your ISPs network BUT because it's actually RF the FCC regulates it (for good reason I guess lol).