I’ve noticed on social media that Austrian German speakers write German differently to represent more of the Austrian dialect. I moved to Austria recently and would like to engage more with the language but I find it very difficult to read and understand written Austrian German online. Are there any rules or patterns that could help me?
The examples you give
Nah wor a spaß wegn am sommernachtskonzert
host du (for hast du)
i (for ich)
kann ma wer sagn (for: Kann mir wer [jemand] sagen... )
are some form of Austrian pronunciation of German. (There are many forms of pronunciation in Austria's various regions, there is not one single "Austrian" pronunciation), and people try to approximate spelling to how they are speaking.
There are no "rules" for this.
There are of course regularities of phonetical transformation. Knowing these regularities would allow non-Austrian-speaking person who has a good command of standard German to more or less authentically imitate the local form of Austrian, but still locals would probably hear that this is made-up Austrian.
Your way to learning this (if you really insist; I do not see the need or use of it) would be: learn standard (written) German well; learn also to speak German with your local Austrian pronunciation. Then write as you find appropriate to highlight the different pronunciation.
A way to learn understanding such written dialectal forms would be: Read it aloud and try to listen to it as if you where listening to a local person speaking Austrian dialect. Try to guess what this would be in standard (written) German.
Perhaps one could phrase a rule as follows: Start from standard spelling. Change those parts that are in dialectal pronunciation most noticeably different from standard pronunciation. Leave the rest (that is more or less similar) untouched.
For example, if you exaggerate, your sommernachtskonzert (which is standard spelling except for the missing capital letter at the beginning) would become something like somanochtskontseat (I tried to approximate dialectal pronunciation maximally). You can do this, but obviously people who use dialectal spelling avoid such heavy distortions because then reading becomes difficult. Host instead of hast du is easy to read. It is a frequent word. Sommernachtskonzert or somanochtskontseat is not.
The orthographic rules for German German, Austrian German and Swiss German are the same, as long as you speak of the standard variations of German language. There is just one exception: In Swiss German there is no »ß«. You need to replace is with »ss« (»Maß« in Germany and Austria becomes »Mass« in Switzerland.)
There are no further exceptions in Orthography for standard German. There are many differences in Grammar and vocabulary between the three German standards, but Orthography is the same in all three variations.
But I guess you are not talking about Austrian Standard German (which is different from German Standard German only in grammar and vocabulary, but not in orthography). I guess you are talking about colloquial speech spoken in Austria or even dialects.
But here is the point: There is no standard for non-standard German. And colloquial speech is non-standard German. So, there is no standard for the writing of colloquial speech. There also is no standard for writing dialect speech.
You can write phrases and words that only exist in colloquial speech, but not in standard German as you like. I wrote about this in a different answer (in German language): https://german.stackexchange.com/a/38520/1487
An example from this German answer:
In Austria you often will hear the word »Oida« (watch this video: How to speak Viennese using only one word). It is derived from »Alter« (»the old one«), but it has no standardized spelling. If it is written, you will find many different forms. You will find:
Non of those forms is wrong, they are all in use.