There are many reasons why you should use a domain name.
TLS is one problem as you already point out. Even for a static site there are reasons to use TLS.
- TLS is not only about protecting the confidentiality but also about protecting the authenticity. Examples have been reported of lack of TLS being used to inject ads. And it could potentially be used to inject malware into static sites.
- Even if the content of the site is public users might not want every network they traverse to know exactly which pages they are viewing.
- Even a static site can have areas which require a link to access. (This might not apply in your particular case.)
There are reasons besides TLS for why to use a domain name.
At some point the IP address may need to change. That will be a lot easier to manage if you are using a domain name and just need to update a DNS record. Otherwise you will have track down every link to the IP and update them. Some of those may exist in users bookmarks, which will be difficult for you to update.
Using a domain name with DNS means you can add redundancy as needed. If there multiple DNS records pointing to different IP addresses a browser will try them in turn. You can't get that kind of redundancy with a hardcoded IP address.
Even if the site has no redundancy it will not be sufficient for you to remember one IP address. You will need to remember both the IPv4 address and the IPv6 address.
Don't assume you can avoid that problem by only using IPv4. By now 45% of internet users have IPv6 and some of those don't have IPv4 anymore. There are workarounds for accessing legacy sites from an IPv6 connection, but some of those workarounds only work for sites with a domain name.