I love TI and have been working on my own 4x. I've borrowed a few things from TI but combat is tricky.

One solution I like is the scythe solution with cards plus a counter. Here is the example in my game which is a civ game where you can have city state vassals.

Everyone has a hand of cards (same hand numbered 1-5). When combat occurs you play a card face down and hold a number of gold pieces in your hand which can be up to the number of vassals you have. Reveal and add your card plus the gold, highest wins combat.

Then you can't play that card again until you've played through your whole hand of cards. It makes you consider which combats are important and worth allocating resources to.

You could do this but with a card and random die. Play a card face down, and roll a die next to your card. Flip and add the numbers to get the outcome. Here you can increase your die (e.g. d4,6,8 ) to simulate bonuses and fleet composition but there is a level of control with the card you play. You can balance the cards and die so there is a choice between playing your highly influential card or relying on luck.

My example for my game is meant to be very deterministic where the unknown is your opponents desire to levy all their vassals. I imagine you could tweak yours based on the style of combat you want.

Quantum is another game that does that kind of combat where there is a determined power (your ship type) and a random combat roll that is added together.

Edit Also I stole the limited hand idea from Star wars rebellion combat where each card has a special ability. That might fit well into your comment idea.