We upgraded straight from M5 to M7 (Windows servers but don't see why it would be different for any other OS). We did it with help from a specialist from SAS though.
And if you're not in a rush, some tech support notes suggest M8 is likely to ship before the end of 2022. (I haven't seen an official release date).
The Boston Area SAS Users Group is hosting free webinars! Next up: Joe Madden & Joseph Henry present Putting Power into the Hands of the Programmer with SAS Viya Workbench on Wednesday Nov 6. Register now at https://www.basug.org/events.
I assume you want to do an upgrade in place so you will need to bear in mind that SAS will be out of action for the duration of the upgrade. Typically this is a multi-day job so your management need to be OK with the downtime.
The SAS Users Group for Administrators (SUGA) is open to all SAS administrators and architects who install, update, manage or maintain a SAS deployment.