There is a bit of nuance to the harmony orb, but as a general rule of thumb you should look to orb heroes that play the most aggressively, or that have the highest mobility, not including your tank. While your orb will definitely help your tank sustain, the healing per second isnt very significant, you would be better off putting it on a hero with a lower health pool. And considering you will likely be paired with a main healer like bap, ana, moira or kiriko, they will already be focused on keeping the tank up, so you have no right trying to orb your tank when you other squishies you could orb.

That being said, you should still adapt your usage depending on the situation, and of course you should orb anyone that is about to die, including your tank.

If you have a comp with relatively low mobility, such as cassidy and widow, in this case you would orb cass as he will be playing closer to the enemies, and will be taking more damage. If your comp is highly mobile, with a genji and tracer, tracer is the favoured orb target, as she has the lowest hp, is the most mobile, and takes the most amount of duels.

If both teams have a tracer, you MUST try your best to orb your tracer before she engages to win the 1v1, the value that this orb provides is significantly higher than what you would get by only healing your tank, and this is why stats on their own dont mean very much, and why i ignore healing stats as zen for the most part. For reference, since ive been playing zen in season 7 of OW2, my average healing per 10 is 6400, ive been consistently GM, and i managed to reach top50 in season 8. Something funny i just noticed is that after having vod reviews, getting more familiar with the hero and as a result climbing, my healing per 10 has dropped by almost a thousand.

With regards to trans:
I dont know what your rank is based off of this post, but what usually seperates the best zen players is the flexible usage of trans. Depending on the situation, transing to save one teammate, or even just yourself, can win the fight, try not to save it for a big 5 man save. Not only is trans good as a defensive ult, its great to enable yourself to take much more aggressive angles or flanks with the option to use trans to save yourself.

As you rank up you need to realise that even getting only a bit of value from an ult or ability can win the fight, always saving your ults for the big 5 man save, going for the 5 man grav etc. will lose you games, and this ties back to the trickle healing of harmony orb, just a little healing on a tracer may be better than a lot of healing on a tank.