Wanting to translate “That was good of you” I wrote: Roedd hynny’n garadeg ohonoch chi”, but said “Roedd ‘na garadeg o’ti”
The written version seems stilted and the spoken a bit sloppy. Any advice please!
Your first one doesn’t sound stilted at all - all fine! (except it’s garedig)
Your second one is wrong, because hynny (and therefore hynna also) can’t be shortened to 'ny or 'na when the hynny or hynna is the subject of the sentence, which in your example it is!
And o’t ti IS a bit sloppy, because even in speech we normally have the -hon-, or at least the -on-, element with this preposition…so onot ti (or onoch chi) would be fine. 
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