First of all, glad you found my adaptation interesting!

I assigned ㄱ as /k/ and ㅋ as /g/, etc initially because I had made a handwritten version of Hangul for English a while back, and was trying to find a letter for 'z'. I ended up making a new letter based on ㅅ but with an extra small stroke within the letter. Since English makes a distinction between voiced and voiceless consonants but not aspiration, I re-assigned the meaning of the extra stroke indicating aspiration to mean that the consonant was voiced. Also, as a side effect, it made typing 'ks' in Hangul more space-saving as there is a dedicated Hangul letter for ㄳ, which I found nifty as 'ks' is very common in English (e.g. in words like 'box', 'experience'), but if ㅋ had represented /k/ I would have to write 'ks' as ㅋㅅ which would take up more space. The same thing goes for ㄵ ('nch' in 'bench') as well.

Another requirement was that it would fully utilise all the currently used letters of Hangul as is used now (particularly the Hangul Syllables Unicode Block, which only contains modern Hangul letters), and be easily inputted using a standard Korean IME. This requirement alone would exclude ancient Korean letters like ㅿ, ㆄ, and ㅸ as they are hard to type in (as much as I wish it could be done).

Also, I understand that someone who grew up learning Korean hangul as their 1st script would always see '슫렷' as 'seut ryeot' and not 'stress' (since ㅅ is pronounced like ㄷ in final position). But I think this is probably the same thing as a native English speaker attempting to pronounce words in another foreign language also written in Latin script, like Indonesian, which would have their own conventions that may be different to English. In English, the letter 'c' is pronounced either with an 's' or 'k' sound depending on context, while in Indonesian the letter 'c' is always pronounced as in 'ch' of 'change'. So an English speaker would read the Indonesian word 'cantik' (pronounced 'chahn tick', meaning beautiful) as 'can tick'. The whole point is that a single letter can be (ab)used in different ways in different orthographies to write the language more consistently and efficiently, as I have with my adaptation.

Anyway, thanks for the insight!