The lever is not camming into the groove in the bolt extension because the cocking piece is not aligned, it is sitting just a touch too far forward.

Could be due to a short (reground) sear, or a drastically stoned cocking piece bent.

Usually on rifles assembled using mix and match parts without consideration to selecting ones that work with each other.
If you had a bin full of spares, switching out components with trial and error would fix. Trying another sear would be my first. Find a brand new one or as close as can.

You need tools, such as a firing pin wrench to strip the bolt completely. A work around with what you currently have, is to stone the sharp front edge of the groove in the bolt extension a few thou to let the edge of the safety lever slip in and engage the groove. Don't mess with the lever's edge.

Hand fitting of the bolt, sear and cocking piece should give a slick safety lever that can be engaged and flicked off with one finger.

You mention that you have to pull back 5mm? That is a huge amount. sounds like you might need to find replacement parts, something very wrong here.

If I had it in my hands I could tell you what's up in seconds. Can you strip the wood off your rifle and take picks of the sear from underneath the receiver, looking up into the action with the rifle cocked?