Not fully, there are still places a backdoor could be hidden (and that's disregarding the possibility of backdoors in OpenWRT, which just recently fended off its own supply chain attack), but I'd sure trust it more.
The thing to keep in mind is that the more sophisticated and difficult to detect a backdoor is, the more valuable it is. And therefore, the less likely it is to ever be used against a normal person. So getting rid of blatantly buggy and insecure software, which TP-Link unfortunately has a bit of a reputation for, goes a long way. And not to pick on TP-Link, evidence suggests many/most home routers are riddled with vulnerabilities.
And to complement your answer, the place where a custom firmware might still be compromised is mostly in the binary blobs, where proprietary code for the radios and some other chips aren't open-source and act like some kind of black box between the software and the hardware and make it work.
Unless someone reverse-engineer those blobs and make an open-source alternative.
Hardware backdoors are also possible in the silicon, and are probably some of the most dangerous. Fortunately also probably some of the most sophisticated and difficult to introduce.
If you can, sell it and buy a low-power used mini or micro PC with two RJ45 ports or the ability to install a second network card. Install OpnSense on it and buy a standalone WiFi AP.