@JasonWalker I finally got time to work on the actual offers last night and unfortunately it doesn't seem like adding dashboard: to the category hides it from the offers tab any longer.
I was able to confirm that the correct format for the relevance statement is
concatenation of trs of (td of html ("<a href=%22takeoffer:" & item 0 of it as string & "%22>" & item 1 of it & "</a>")) of (id of it, value of header "Subject" of it) of actions (ids of fixlets whose (value of header "X-Action-Component-Type" of it = "Action") of sites whose (type of it = "Master Action Site" or type of it = "Operator Site" or type of it = "Mailbox Site")) whose (offer of it and pending of it and value of header "offer-ui-category" of it = "dashboard%253aMyHiddenDashboard")
I did find a reference to a forum post where this was discussed in 2022 and apparently it wasn't working then either.
Self Service Technician Tab HTML - BigFix Forum