So I've been a developer at my current job for about 12 years. I am the most senior level developer at my job. Let me state that I am a backend developer although I did frontend development off and on as well for the first 5 years of my career. However I have done no major frontend development for around 7 years now.

Effectively our frontend developer of 6/7 years just left.

We had an existing project in the queue and my boss expected me to do frontend development for this project which I did just to help out, but I am not getting any extra pay for this and I absolutely hate doing it. The only thing I was paid for was I overtime for completing the project quicker. With that being said I feel like I should be paid substantially more since I am doing double work and since they are not paying for a frontend developer. I'm literally doing her job and doing a better job than she did mistake wise doing her job.

Additionally many things have changed over the past 6/7 years and they have it in their minds since I did it in the past it should be the same now which isn't the case. So there are things in my project queue right now for future projects that they think I know how to do and I don't. It isn't that I couldn't eventually figure it out. It is just that I have zero desire to learn it .I just absolutely hate styling websites.

I'm ok with doing minor frontend things for projects but not entire websites

I literally develop the backend off all the sites we build setup Google tag manger tags/triggers, Google analytics, search console, Google looker studio, dns, site updates, manage all out Linux servers, do seo for content and sites. I can't handle something else on my plate. I'm currently having to rewrite a ton of code as well due to upgrades for our sites.

How do I respectfully tell my boss I refuse to do frontend work going forward or pay me substantially more on another project and that he needs to hire someone else without damaging our relationship?

I like my boss and my coworkers as people a lot outside of work, but I feel like I'm being taken advantage of financially and I'm honestly tired of it. As a developer for 12 years I'm honestly ready to just go elsewhere.