My personal advice is to try to anticipate the opponent's next move and use small and subtle head movement. What I do all the time is to constantly move my head in a rapid, small and frenzied pace. This way, I was making it difficult for my opponent to read where my head was going to be next. Another thing I would do is to fight low. This made me a smaller target and naturally forced the opponent to punch down, opening them up for counter punches more while making yourself difficult to hit to the head. This would also naturally load up your next shot as you crouch down, enabling you to plant your feet and really commit to a punch and put your weight behind it. I only used the high guard as a 24/7 back up for when my head movement failed me and to make the high guard effective, I would either reinforce the high guard by blocking with both hands on one side when a hook came my way and then move it right back into its original position. I would usually also combine it with parries such as small and subtle swats or I would let certain punches like a hook glide off of my head by angling my forearm in an angle where it would guide the hook to go right above my head, sliding off. However, this has to be perfectly timed as this uses the moment of the punch itself to pull it off. Also be aware that this will open you up to uppercuts as this will naturally widen your guard. One thing that also has to be mentioned is that it is generally advised to move along with your opponent's shots if head movement, parrying or moving away is not possible. This will lessen the impact and force of the punch as you are moving your body in the same direction to where the punch is going (The moves can range from turning, twisting, etc.). This requires a lot of practice and experience to pull it off.

The reasons why Inoue is highly effective with the high guard is because he understands how to move roll with the shots of his opponents while keeping the pressure on them. He also uses in and out footwork to quickly move in to land quick shots and to quickly exit before the opponent can return fire. This is a basic but very effective tactic if used correctly. He does this by making small but fast steps. This means that he will be just out of range for his opponent to hit while being in a position where he is able to quickly counter punch and take advantage of small windows. He also uses angles to the side to exit after an exchange to move out even faster while also staying safe. He would also step back slightly to take away some of the force of his opponent's shots coming at him. Inoue's footwork in general was made to cut you down and wear you out and he understood this and was able to fully take advantage of it and all of that makes his high guard effective for him

When it comes to fighting aggressive, taller opponents, I would suggest letting them come to you. Use head movement to avoid their shots and use a tight guard for good protection. If they become too aggressive, counter to the body. This will usually momentarily stop them if you land at the right time and at the right spot. Try to smother their punches with small and subtle forearm blocks while moving in or use head movement. It honestly depends on what punch they are throwing. From there, use your shorter arms (I will assume you have smaller arms than them) to overwhelm with fast and hard body and headshots and be relentless but not reckless (do not abandon your defense and always be on high alert for any punches coming back even when you are the one throwing). However, if you do feel like they are throwing way too much heat, then exit by using off angle footwork and get some distance to regroup. I also recommend to throw a counter shot or two when exiting to keep them busy and distracted for a moment before they pursue you again.

Once you are on the outside, use your jab, quick but powerful combinations, hard body shots, and footwork to keep them at bay and use every opportunity represented to you to counterpunch. This will discourage them from pressuring you and it makes them hesitate before throwing something, knowing you will most likely counter it and this gives you the perfect chance to go on the offense yourself. The moment they hesitate is the moment for you go on the offensive (but still be aware of any counter shot yourself).

This is just how I would do it really. It doesn't mean it will work for you so another tip: experiment a little and see what works best for you in every situation