What you have so far is pretty good! Thinking specifically about SEO, with a super-niche topic like this, it can be hard to find the keywords that people are searching, but focusing your content around a few key phrases is important. I did a bit of keyword brainstorming...
Unfortunately, the word "letters" has multiple meanings, and some of the people searching "old german letters" may be looking for info on alphabet characters - some of the more common modifiers people are using with the phrase seem to indicate they are talking more about the alphabet characters:
old german capital letters
old german cursive letters
old german letters keyboard
old german letters of the alphabet
You might consider using "documents" in place of "letters" in some places - with the phrase, "old german documents" as a root phrase, I see a little traction with terms like these:
translating old german documents
old german documents ww2
translation german english documents
translate german documents needed
translate scanned documents german english
german documents translated into english
There might also be a bit of search volume around "handwritten" terms, I am seeing:
translate handwritten german to english
translate handwritten german
translate handwritten german to english
There are a few places you'll want to make sure to use your chosen keywords - in the page title (<title> tag), and in the main heading at the top of each page (<h1> or main heading tag). If you feel like doing some reading, the Moz beginner's guide is great (
https://moz.com/beginners-guide-to-seo).
This chapter has specifics on keyword usage and on-page optimization that you should definitely apply.
There are dozens of other factors that can have small effects on rankings (mobile-friendly layout, using an SSL certificate, adding a privacy policy page, changing image file names to include keywords, etc.), but the biggest impact beyond good on-page optimization with well-targeted keywords will be incoming links. If you can encourage related websites around the web to link to the domain, you will see rankings for related keywords increase. "Building" links to a website is one of the hardest and most controversial aspects of SEO (there are more chapters in the linked Moz guide about best practices in link building).
I love AngryOcean's ideas for other marketing and outreach - the more exposure you can get the site on forums and related websites, the more likely you will gain some valuable links to the site. It will be important to avoid "spamming" communities with promotion, and would be more effective to ease in as a genuine participant. Manually promoting the website/service will be time-intensive, but maybe your mom is savvy enough to start participating in a few communities related to genealogy and family history? SEO helps you rank well for people who are already searching your service, but there's a whole world of people out there that don't yet know they want the service, as well. Building relationships that lead to promotion in the right circles could be very impactful.
P.S. most/all search engines these days completely ignore the "keywords" meta tag, and some (Bing) have even said they penalize the rankings of a website if they are using that meta tag, so I'd recommend removing the meta keywords tags from pages ("<meta name='keywords'...>").
