@TheJudge91 youre copy pasting issues mentioned on other sites, you should look at the reality (im no genius but look at my implementation above)
back button does not break because im not navigating the iframe, im navigating the whole browser tab
iframes work fine with screen readers - "Generally, using iframes won’t negatively impact the experiences of people who use screen readers and other assistive technologies. However, iframes must have meaningful titles." https://www.boia.org/blog/why-are-iframe-titles-important-for-accessibility
css styling works fine on the iframe, yes there are exceptions but they are not needed here
i very much doubt performance/memory is a problem for a simple header menu
apart from using static page generator like @maks112 mentioned, i think its the best option for a static website without javascript
but since OP returned and said he changed his mind and going modern with javascript, then framework makes the most sense right now