I am running a relay with the tor-0.4.3.0-alpha-dev distribution, which I installed as a noob mistake. Is it okay to continue with this version? If not, then do I just do a standard apt-get uninstall and install a more stable version? If so, which? I have already changed /etc/apt/sources.list to use ...torproject.org bionic main.
Yes, it's absolutely fine to run a alpha version of Tor as a relay if and only if you're a relatively advanced user.
First of all, alpha's need testing. However, be prepared for bugs and please only do this if you're also going to report the ones you encounter too.
Secondly the Tor network also gains resilience from different Tor versions running on different OS and hardware platforms. It helps diffuse the possibility that any one bug affects the entire network.
You can see in the Tor consensus document, that some alpha versions are defined as recommended versions for servers (I.E. they are considered suitable for use as relays).
server-versions 0.2.9.15, 0.2.9.16, 0.2.9.17, 0.3.5.8, 0.4.0.5, 0.4.0.6, 0.4.1.2-alpha, 0.4.1.3-alpha, 0.4.1.4-rc, 0.4.1.5, 0.4.1.6, 0.4.1.7, 0.4.2.1-alpha, 0.4.2.2-alpha, 0.4.2.3-alpha
You can easily check the current list at: consensus-health.torproject.org