If you haven't already, I would suggest reading Warlock of the Magus World. The MC is very opportunistic like Fang Yuan, and although there is technically romance, it is a small part of the novel and he enters relationships for pragmatic purposes rather than out of love.
Fang yuan is one of a kind. "It's lonely to be invincible" the MC's ruthlessness and cunningness is quite similar thou the other mc considers himself to be righteous
Ah, a youngin. Firstly, check out this list: https://www.novelupdates.com/viewlist/20556/ It contains several novels that'll offer you a similar main character to Reverend Insanity along with links to several other lists which contain other novels as well. You can also check out my list, https://www.novelupdates.com/viewlist/31711/, I'd say its alright but it also contains a few novels that I've not read. I'd recommend you to read Soul of Negary.
Infinite Bloodcore Well its basically from the same author.. if what you are searching for is actually an mc that stick to his goal and believe in his judgement though he the mc here is the complete opposite to Fang Yuan. unlike fy that pure evil this mc from Infinite Bloodcore is like Geralt from the witcher.
Dunno, if it was so easy to find MCs like Fang Yuan, RI wouldn't be so special. A few would be what @Despapa69 said, and Way of the Devil, Nightmare's Call, Floating Immortal Order, and Nightfall.