I'm not understanding what your obsession with your enemies feeling pain - or at least eternally ongoing pain - upon being excluded from the coming Kingdom is. What do you care, on a personal level?
I mean, Matthew 18:8 is easily reconciled with the annihilationist (or conditional immortality, whichever way you prefer to label or view it) position, as well as the ECT position. It is a verse about punishment by fire, and being eternal, that fire obviously comes from God. He's almost always used fire to destroy or purify throughout the Old Testament, and there's no reason to believe He'll do anything different when punishing unbelievers for the ultimate and final time before making all things new at the Great White Throne judgment. Whether you personally believe that the fire in Matthew 18:8 will be "only a couple of seconds of pain and you'd be annihilated", or just as lengthy and intense as that pain which our Savior suffered before His death on the cross (which I personally believe, because I see no reason to think of Jesus' time spent in great pain and humiliation on the cross before breathing His last as anything but a substitutionary punishment, but that's another discussion), or eternally ongoing without end, the message of Matthew 18:8 remains the same: even if you were to be literally maimed in order to have eternal life with God, it would still be better than meeting your end in His eternal fire.
Let's just go ahead and assume, for the sake of argument, that Jesus literally meant you would live forever with an eye plucked out or a hand amputated, if you had to do that before entering eternal life. Like, your eye or hand would never be restored. ... I would still rather have that, much rather, than lose my life forever in the second death in the Lake of the Fire, with no hope of resurrection ever again, no matter how short or long the pain of burning in the fire would last before actual death. Just because Jesus used the expression of maiming yourself physically in order to
escape the eternal fire, doesn't mean that the only worse alternative possible to the maiming, then, is to suffer the pain of burning forever without ever actually being consumed by the fire.
You're absolutely right. In fact, He was sweating great drops of blood in fearful anticipation of it, if I correctly recall the wording used in the gospels to describe His feelings about this while praying in the garden. But He did eventually die on the cross, didn't He? (The only difference between He and unbelievers being that He would resurrect from death, and the second death in the lake of fire will quite self-evidently deny them the chance.) If the ultimate fate of unbelievers in the lake of fire is anything like the six hours of pain and humiliation that Christ suffered before actual death, with never a chance to come back to life and have immortality forever like those who trust in Him will, then I want no part of it. A literal second death in the lake of fire is a fate I would still be greatly fearful of. Might be a lot better than eternal torment, but still a fearful punishment as far as I am concerned.