如是者 seems to be a modern day Cantonese phrase:
so; so that; in sequence with the above mentioned
你而家開始一個月儲三千蚊, 如是者, 三十年後你就夠畀首期。
You can try to save $3000 a month from now on, so that you can afford the down payment after 30 years.
I'd treat it more as a set phrase rather than as its individual components.
穷索冥搜，忘寝废食，[如是(者)]有年 = searching earnestly, to the point of not sleeping or eating, and [like this (way)] for years
如是(者) = like this (way) --> "way" refers to " in the manner of (穷索冥搜，忘寝废食)"
You can see '者' as the pronoun for 'the not sleeping or eating way of search'
他日间工作夜间读书，十年如是 = he works at day, study at night ...