Is there a paradigm to form the future imperative?

First, a few words of caution: When learning conjugations (and declensions), I very warmly recommend learning which vowels are short and which ones long. What you've learned is true with the correct vowel lengths, too, though, but it pays off to be careful. Also, the forms you list are the singular ones only. For plurals you mostly add -te, but remember to change a short e to a short i. And there are irregular imperatives like fac, dic, duc and verbs like velle and meminisse that go entirely off the rails, so don't take patterns like the one you learned as absolute facts.

First, here are the singular and plural forms of present and future imperatives for some regular and common irregular verbs:

In the future, the only difference between singular and plural in the second person is the added -te, and this is uniform across conjugations as far as I know. There is the additional third person plural form, which I listed in a separate column.

The way you choose to describe these is up to you — the pattern is in the eye of the beholder. You can just learn the tables by heart, or you can come up with a rule of thumb and acknowledge that it may come with exceptions.

For the singular present ones, your rule of thumb is fine. For plurals it suffers from the fact that you have to remember to switch vowels. Perhaps you can memorize that the plural one is like the corresponding present indicative form, with -tis replaced with -te.

With all that out of the way, here is how I would describe the future ones:

• The third person is simple: Just take the corresponding present indicative third person plural form and add -ō.
• The second person is trickier: If you take the corresponding present indicative third person singular form and add -ō(te), you are otherwise correct but the vowels before the final one are too short in conjugations 1, 2, and 4, and for īre. A more reliable rule is to take the present indicative second person plural form and replace the -is with -ō(te). This one gets all the vowels as they need to be.

While those work, I don't find them very useful. You have to remember that somehow oddly you use the third person plural for the third person ones (which sounds sensible) and the second person plural for both singular and plural second person ones (which is odd). I would definitely struggle to remember that right.

To me, at least, the superior way is to just learn the tables and observe some regularities and develop a nose for things that may go wrong (like specific vowel lengths or changes) and memorize exceptions.

If you want to come up with a rule, anything is fine as long as it correctly gives you all the forms in the table above. A rule that fails to reproduce those forms is not very useful and is more likely to hinder than help.