For my benefit, and that of others who may be curious about this, please help me understand where this type of setup falls into this hierarchy:
1.) Regular consumer routers with manufacturer firmware (e.g.: the typical TP-Link, Netgear, ASUS, etc.).
2.) Regular consumer routers with 3rd party firmware (e.g.: open source projects - OpenWRT, DD-WRT and FreshTomato).
3.) People who customize their personal computer to have router functionality (I don't know much at all about that; a little searching suggests DD-WRT and pfSense can be used for the software).
There's less difference than you might think between these options. In the old days, routers were just computers (often, but not always, running a Unix operating system) that ran a software program that could "route" network information between two networks. They also often included some sort of firewall software. Aside from the software, there was nothing really special about the computers, except that they generally had two independent network ports to allow connecting to more than one network at the same time.
That is basically what you see with today's consumer routers. They're just simple, low-end computers with two or more network ports: one port for connecting to a wide-area network (WAN), e.g., the Internet, and one or more ports for connecting to the local-area network (LAN), e.g., your home network. Most of them are running some type of Linux, but most of the general purpose Linux functions are turned off or locked down, and an administrative interface is exposed to the end-user to allow limited customization, usually via a web page or a mobile app. Generally, consumers are able to access only the functions presented through the official administrative interface, though sometimes it is possible to get access to the underlying operating system.
Regular consumer routers with 3rd party firmware operate on the same principles. The main difference is that the firmware is a different version of Linux (or BSD) that is supported by the community, so it often is maintained significantly longer than the router manufacturer's original firmware. It also tends to get security updates more frequently. Another important difference is that the underlying operating system usually is accessible to the end-user. This allows a lot of flexibility, but it comes at the potential of greater complexity. Much of this added flexibility will be available via a web-based administrative page, but you can also access the underlying operating system via the command-line and customize it much more extensively.
For example, software like OpenWRT will have most services turned off or locked down by default, but you can login to the system and add just about any standard Linux software to the router, assuming that the router has enough RAM or disk space to run it. A common example of this is someone who has a consumer router with a USB port, and they'd like to use the port with a USB drive to support Time Machine backups that are compatible with the upcoming macOS 27 release. Make a few configuration changes, and you are in business. Another common option is to enable USB printer sharing.
The final option you suggest really is no different from running a consumer router with third party firmware, except that you have the option of even greater customization, especially at the hardware level. For example, if you have your own small business, you might try installing several network cards in a PC and using the PC to route traffic between three or more networks, including wireless networks. You might add more RAM or storage to be able to support more devices or users. In many ways, this is the most traditional approach to setting up a router.
Each of these approaches has their own limitations.
Regular consumer routers with manufacturer firmware are very easy to use, but they at the mercy of the manufacturer in terms of security updates and features. They generally receive fewer security updates than other types of routers, and manufacturers often will stop supporting them for business reasons, rather than for technical reasons.
Regular consumer routers with 3rd party firmware can be great, but the assumption is that the end-user has more technical skills than most consumers. Installing third party firmware is not very hard, but it is more demanding than simply updating the manufacturer's original software. Also, once installed, there usually are a few configuration steps that are required. Again, not terribly difficult, but if the end-user knows little or nothing about networking, it may be a bit beyond reach. Similarly, if an end-user wants to customize the installation much beyond the defaults, they can find themselves in a place where it is easier for a novice to reinstall everything from scratch than to try to figure out what went wrong. If you are comfortable with Linux or BSD and have an understanding of networking concepts, it's a great solution. If you are a hobbyist looking to learn a bit more, it can be an excellent fit. If you're a regular consumer, make sure to read the installation instructions to make sure you are comfortable with them, or maybe find a friend who is a bit more technically inclined who can help you. As you might imagine, the same guidelines apply to trying to use a regular computer as a router.
I don't want to scare anyone away from trying to install 3rd party firmware on a consumer router or to use something like pfSense on a PC. It can be challenging for novices, but if you're careful and follow the installation and configuration instructions closely, it can be a very rewarding process. That said, most consumers probably are better off simply buying a currently supported router from a respected manufacturer.