Yes, the root groups remain the same without the "parent" tag, it should only exist in the sub-groups, it may be interesting to use a "depth" tag to know the level of the group in the structure:

Root Group (depth = 1)
"tags": [
    ["d", "tech"],
    ["name", "Tech"],
    ["depth", "1"],
  ]

Subgroup (depth = 2)
"tags": [
    ["d", "nostr"],
    ["name", "Nostr"],
    ["parent", "tech"],
    ["depth", "2"],
  ]

Sub-subgroup (depth = 3)
"tags": [
    ["d", "nip29"],
    ["name", "NIP-29"],
    ["parent", "nostr"],
    ["depth", "3"],
  ]