Ok here are some conclusions I have made but happy to reconsider and get it right. Hence my posting here and asking for advise.
1. I am not able to find appropriate powder coating material here in NZ. Not even sure if "powder coating" is the right term for the dry powder tumbling and baking process on Youtube. Is that Polymer coating? I might just buy the Missouri bullets 340 gr from reloading international if I need to do that - box of 300 costs about US$54 and then I need to pay $90 for export & freight. Still worth it I guess. Hornady pistol jacketed 400 gr bullets cost US$60 per 50 pieces here.
2. I want low recoil ammo. I shot a 470 NE Chapuis once that did not fit me and it really hammered my face. I had a really sore cheek bone for 2 weeks. On the other hand a 600 NE Jeffery full house load of 900 gr at 1850fps pushed me back half a step. I shot my 416 Rigby ok with 400 gr bullets at 2550 fps but found the recoil quite heavy. But I shot the same combination at 2400 fps really well. I found a BIG change in felt recoil and rifle control, particularly off hand rapid fire.
3. Therefore I plan to load Re15 - well established loads - 85gr to 87 gr is what is most commonly suggested as regulating loads. The 75% rule is what the 400 gr cast or pistol bullets give me for regulated loads at lower recoil. Federal ammo is 87 gr Re15 without filler but using 216F primer which are hotter and not available to reloaders.
4. So I plan to use foam backing rod for jacketted bullets with Re15 & F215 primers as many people do. Some do not bother with fillers and others use dacron. (See earlier comment on barrel ringing).
4. My goal is to achieve that without barrel leading. 3 conclusions
(a) Hard cast - wheel weight + 50/50 solder and water quench
(b) Felt wad lubricated to minimise leading (Will foam backer rod filler work for cast bullets?)
(c) Card wad to prevent felt from sticking to base of bullet and throwing the shots wild and also to prevent powder contamination by the lube
This is the theoretical conclusions I have come to so far, even before casting a single bullet or firing a shot. Heck, even before seeing the rifle and holding it in my hands! I think this kind of research and analysis minimises time and effort in achieving my objectives. I hope I am starting with correct assumptions for fine tuning.
Any challenges, criticisms and corrections are welcome.