almost alway rokssolana. only a few verbs have kept their imperative form. instead we just use the equivalent future form. you were also right about the three forms, since in hebrew we use the imperative only for you: masc sg, fem sg, or plural. this form would usually be pronounced te-he-nu and not te-ha-nu in speech.
From Reverso language app: conjugation section: תֵּהָנֶה/תֵּיהָנֶה , There's a lot more; these are the 2nd person, future tense (The word to the left of the forward slash shows the variation. Transliteration is theirs). Here's the chart on their website: http://conjugator.reverso.net/conjugation-hebrew-verb-%D7%AA%D7%94%D7%A0%D7%99.html
And the same from my 2nd fav resource: https://www.pealim.com/dict/465-lehanot/
תֵּהָנוּ ~ תיהנו tehanu
Future tense, 2nd person, masculine, plural:
you m. pl. will enjoy
Thanks can't have an apostrophe, unless someone's name was "thank" or "thanks." It's either for possession ('s cat) or shortening from, for example, there is = there's. Wrong = Thank's, because it can't be 'thank is'. So if you have a cat named thank; you could use the apostrophe (example: Thank's litter box needs cleaning). There's a lot of contrary information about English rules, but if you need more info I recommend: https://www.grammarly.com/blog/apostrophe/ (where they explain the rules and if there are different opinions, your options).
As dovbear57 wrote, in most verbs Hebrew speakers prefer the future form, and the imperative sounds very formal. With this particular verb, it's one step beyond: the imperative is not in any kind of use. Most native Hebrew speakers, if you ask them "what's the proper imperative of תהנו", would be puzzled for at least several seconds. Then, they will realize it must be ההנו (at least those among them that are language-minded will realize that), and then - it should be fun to see their faces, because it sounds so terribly... wrong...
Now, what would a writer or an editor who insist on proper Hebrew would do if pushed into this corner? Surely they'll just just side-step the issue by looking for another verb.
It's not a question of correct or formal Hebrew versus colloquial Hebrew. Though what you are saying about this particular verb makes sense, given its meaning. Using the future tense as an imperative has existed since Biblical times. Modern usage has simply sorted verbs into which are most suited to which kind of imperative, rather than leave you to decide at the last moment.
It's true that some verbs are still used in proper imperative, even in everyday speech. But I think formal vs. colloquial distinction is true for the other (majority of) verbs: I'd consider it wrong to see future used in an imperative sentence structure in texts that are formal, that intend to follow "proper rules".