100% agree that with op's existing fixturing it's absolutely a non-starter.Looking at the part and its fixturing, it doesn't seem like the most rigid situation with that stem holding the big wide top, so yeah, a heavy-action cut from an inserted tool is possibly going to be a problem.
I still think commodity 1/2" endmills zipping around at 200-250 IPM is the way to go. 8-10 in^3/min, low impact cut, cheap tooling.
Nonetheless, I have done a LOT of heavy removal parts in 4xx0 steels with 50 taper machines, and as much as I love adaptive paths/high speed roughing, I have never, not once, produced a shorter cycle time on this kind of part with that strategy.
Adaptive roughing is great for pockets and lighter spindles. For 2.5" thick pieces of 15" bar with >50% material removal, there are no endmills cheap enough to make it worthwhile compared to big cuts with indexables.
But for sure, op would have to improve his fixturing.