Try to test the system and review the results using this system status for CPU, Memory...
https://x.x.x.x/check_sys/127.0.0.1.html on the iSCSI server (has this been updated to Version 172 Build 2512) . Check to see if all firmwares have been updated. Some testing on all levels will need to be performed. Again, on the internet there are many areas that talk about these subjects from your inquires of what do they mean " InitialR2T and ImmediateData" .

InitialR2T

The InitialR2T key is used to turn off the default use of R2T for
unidirectional and the output part of bidirectional commands, thus
allowing an initiator to start sending data to a target as if it has
received an initial R2T with Buffer Offset=Immediate Data Length and
Desired Data Transfer Length=(min(FirstBurstLength, Expected Data
Transfer Length) - Received Immediate Data Length).

The default action is that R2T is required, unless both the initiator
and the target send this key-pair attribute specifying InitialR2T=No.
Only the first outgoing data burst (immediate data and/or separate
PDUs) can be sent unsolicited (i.e., not requiring an explicit R2T).

ImmediateData

The initiator and target negotiate support for immediate data. To
turn immediate data off, the initiator or target must state its
desire to do so. ImmediateData can be turned on if both the
initiator and target have ImmediateData=Yes.

If ImmediateData is set to Yes and InitialR2T is set to Yes
(default), then only immediate data are accepted in the first burst.

If ImmediateData is set to No and InitialR2T is set to Yes, then the
initiator MUST NOT send unsolicited data and the target MUST reject
unsolicited data with the corresponding response code.

If ImmediateData is set to No and InitialR2T is set to No, then the
initiator MUST NOT send unsolicited immediate data, but MAY send one
unsolicited burst of Data-Out PDUs.

If ImmediateData is set to Yes and InitialR2T is set to No, then the
initiator MAY send unsolicited immediate data and/or one unsolicited
burst of Data-Out PDUs.

MaxRecvDataSegmentLength

The initiator or target declares the maximum data segment length in
bytes it can receive in an iSCSI PDU.

The transmitter (initiator or target) is required to send PDUs with a
data segment that does not exceed MaxRecvDataSegmentLength of the
receiver.

A target receiver is additionally limited by MaxBurstLength for
solicited data and FirstBurstLength for unsolicited data. An
initiator MUST NOT send solicited PDUs exceeding MaxBurstLength nor
unsolicited PDUs exceeding FirstBurstLength (or
FirstBurstLength-Immediate Data Length if immediate data were sent).

MaxBurstLength

The initiator and target negotiate maximum SCSI data payload in bytes
in a Data-In or a solicited Data-Out iSCSI sequence. A sequence
consists of one or more consecutive Data-In or Data-Out PDUs that end
with a Data-In or Data-Out PDU with the F bit set to one.

FirstBurstLength

The initiator and target negotiate the maximum amount in bytes of
unsolicited data an iSCSI initiator may send to the target during the
execution of a single SCSI command. This covers the immediate data
(if any) and the sequence of unsolicited Data-Out PDUs (if any) that
follow the command.

FirstBurstLength MUST NOT exceed MaxBurstLength.

DefaultTime2Wait

The initiator and target negotiate the minimum time, in seconds, to
wait before attempting an explicit/implicit logout or an active task
reassignment after an unexpected connection termination or a
connection reset.

A value of 0 indicates that logout or active task reassignment can be
attempted immediately.