I have always been able to find a pretty solid fire forming load that was close to my final load.
The cost of a shot is a lot more than the bullet. Assuming 1500 rounds barrel life, if you bought 200 rounds of brass and got 7-8 firings and a rebarrel cost $700 and you can buy all the powder in 8lb jugs at $250 each
Total cost of ownership includes barrel wear, brass wear, and the other components.
$1,160 in barrel and brass (figure $2.30 for quality brass if you can find it.). $3800 in powder and primers you are paying $3.30 per shot without the bullet.
I would not bother with the whole fire forming as its own exercise in this case. Find something that works within reason and fire for effect. it won't take long to get enough brass formed to start working on the primo load and you will also have a head start on your permanent load.
Typical barrel life can look like this in a big overbore: 1500ish barrel life - 200 ff loads = 1300 - 65-100 accuracy development loads = 1200 loads. Start chasing the lands in another 5-600 rounds and then ~ 250-300 thereafter.