I would suggested that rather than just removing some files and disabling services to remove whole packages. If you remove updater then you can use opkg for that, otherwise it would be better to do all that by updater configuration.
I really don’t understand what people have against updater these days. I have just one huge warning: Updater is not there because of our maliciousness but because opkg is just broken in its core design and it will break your system if you do bigger updates with it!
I can understand that you don’t want Foris because you feel that you are advanced enough to do everything from Luci. I don’t think that removing knot is such a good idea and definitely not in favour of dnsmasq. I can see that some people are annoyed by it because of its stricter rules and standard implementation but I still think that it is better choice than dnsmasq. If you want to try something else then you can use unbound in place of knot-resolver. Unbound is supported (not directly on Omnia) because it is primary resolver on Turris 1.x routers.
Now to main thing: For example to remove Foris it is as easy as adding following text to some new file in /etc/updater/conf.d and running pkgupdate:
Uninstall("foris", "turris-webapps", {priority=60})
Note that you might want to add some packages back as there are a lot of packages just depending on Foris and they are removed when Foris is forced to be removed. For example to retain netmeter you can just add Install('netmetr').
The same thing can be done for knot-resolver or any other package. Well there are some exceptions such as updater it self, kernel and core system tools (such as procd). Here https://repo.turris.cz/omnia/lists/ you can see online part of updater’s configuration. Documentation for updater is in our documentation https://doc.turris.cz/doc/en/howto/updater and for advanced usage there is this language reference: https://turris.pages.labs.nic.cz/updater/docs/language.html.