твердая сталь - ОК
сбалансированная справедливость - ОК, but too "scientific". Many people would hardly get the idea what it means Try to find a simpler adjective e.g. всеобщая справедливость
прекрасный спектакль - ОК
опасный зверь - ОК
стойкий портфель - grammatically OK, but I can't get what you mean by this. Стойкий is usually applied to people (firm, steady man). Or to something abstract (stable state).
надежный приятель - OK
осторожный слушатель - OK
умный вождь - OK
яркий огонь - OK
широкая тень - OK
переполненная площадь - OK (you wrote "плошадь", a typo again)
тихая жизнь - OK (corrected a typo)

Impulse, your approach is good!
I would just recommend an easier way: if your purpose is to memorize the gender, it's enough to use some simple pronouns "этот/эта" or "мой/моя". Isn't it easier?

And to make the things even easier, there are some patterns.
100% of nouns with the suffix -ость are feminine. All the nouns with the suffix -тель are masculine. And maybe some more patterns.