For Haven you should try this for Vega56:
"gpu_conf" :
[
{
"id" : 0,
"intensity" : 56,
"worksize" : 8,
"threads" : 2
}
]
No need to change drivers, 18.5.1 is ok.
Also add
--preparedata to your start.bat cmd line, it gives a little boost on Haven for Vegas

I will try it first. looking forward for your brand new SRBMiner Ver. 1.8.9 i have MSI RX580 8GB ARMOR and Gaming X Rigs ready to deploy.