PC6248 Switch to Switch LAG STP Issues with VLANs

I apologise up-front for this question, as it's got to be something very simple to fix, but I can't see the wood for trees, and I'm not familiar enough with LAGs and VLANs.

We have a stack of 3, 6248's connection with their stacking cables, with the default VLAN 1 on all ports. There is then a stand alone 6248P which also has VLAN 1 on all ports.

There are then 2 pairs of cables performing LAG duty between the 3rd switch of the stack, and the single 6248, one of the LAGs purely for VLAN1 (normal network traffic) duty, the 2nd LAG for iSCSI duty (VLAN 50).

My issue is that I need to disable STP at one end of the iSCSI LAG otherwise the LAG is put into standby/discarding mode, as it's seen as being a redundant version of the other LAG.

VLAN1 is removed from the iSCSI LAGs (but not from the underlying ports, does it need to be?) and portfast isn't enabled on the LAG ports.

All ports/LAGs are set to Access (is this the issue?)

The stack is xxx.xxx.xxx.10, relevant(?) config below:

console#show running-config interface port-channel 4
description 'iSCSI SWITCH to RACK 6248'
switchport access vlan 50
switchport forbidden vlan add 1

console#show running-config interface port-channel 3
description 'RACK 6248 1 and 2'
spanning-tree mst 0 external-cost 10000

The single is xxx.xxx.xxx.9, config below: (note STP disabled as it only works this way).

console#show running-config interface port-channel 3
description 'iSCSI LAG3 SWITCH to SWITCH'
spanning-tree disable
spanning-tree mst 0 external-cost 10000
switchport access vlan 50
switchport forbidden vlan add 1

console#show running-config interface port-channel 1
description '6248_SW3_47-48'
spanning-tree mst 0 external-cost 10000

I'm sure the issue stems from the fact the switch-switch LAGs should be setup using Trunk mode, as per the docs, but according to this I'd then need to setup the server iSCSI NIC to tag (which is OK, but would have to be done out of hours)

Trunk: Trunk Ports may belong to as many VLANs as desired. Trunk ports will accept tagged frames only. Untagged frames will be discarded. Frames sent from this port will be sent tagged. Ingress filtering is always Enabled on Trunk ports. Incoming frames will undergo Ingress filtering and if correctly tagged, (tagged with a VID of one of the VLANs to which the port currently belongs) will be admitted. Trunk-mode ports are intended for Switch-to-Switch links, where in general all traffic is tagged.

General mode would also appear to do what I need, but without setting up tagging on the server & storage, would give nothing over Access mode.

General: General ports may be members of as many VLANs as desired. The user may set separately for each such VLAN whether it should be Tagged or Untagged. This setting applies to transmitted frames. Incoming untagged frames are classified into the VLAN whose VID is the currently configured PVID.Ingress filtering may be turned OFF on General-mode ports, if so desired. Ingress filtering is ON by default.

So, is my issue the LAG mode, tagging, or should Access mode work fine and there's an issue somewhere else?

Thanks

Chris