How to migrate from using the Taxonomy Access Control module to the Group module? We have a Drupal 6 site, and we are making use of the Taxonomy Access Control (=TAC) module for access control. We evaluated the migration steps, and even if TAC could be ported to 8, maybe we could use a different approach.

We are evaluating Group as an alternative to content access control, but there are no automatic path to migrate from (as now). What would be the preferred migration path for TAC to Group?
My recommendation: go for such replacement. In doing so, you could use the approach detailed in How to bulk assign content to a group. This community documentation page explains how you can use the Rules module together with the VBO module to bulk-assigning existing content to groups.

Here is the Rules Component included in that page (in Rules export format):

{ "rules_assign_a_node_to_a_group" : {
    "LABEL" : "Assign a node to a group",
    "PLUGIN" : "action set",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules" ],
    "ACCESS_EXPOSED" : "1",
    "USES VARIABLES" : {
      "node" : { "label" : "Node", "type" : "node" },
      "group" : { "label" : "Group", "type" : "group" }
    },
    "ACTION SET" : [ { "data_set" : { "data" : [ "node:group" ], "value" : [ "group" ] } } ]
  }
}


Refer to the community page mentioned above for more details.

PS: the above rule assumes your migration target is D7 (there is no stable Rules release for D8 yet ...). But by using the Group module you're in very good shape also to later on upgrade to D8 also, since this module supports both releases.