Having a User Interface that benefits and assists you is the most important thing to keep in mind for your UI. Making sure it provides you information in the quickest, easiest way possible to read is vital to making decisions mid combat.
As a healer, I would strongly recommend:
Raid Frames: Base blizzard raid frames are great, they work perfectly fine for how you would like. Personally I use Grid2 as it allows me to heavily customize my frames to suit my preferences, and assist me in healing. The Grid2 profile pictured below is available to import from the example UI.
Bigwigs/DBM: Having encounter timings is important for healers to predict damage, position correctly and use appropriate utility during a fight. Having timings will allow you to be more aware and is highly recommended.
Nameplates: There are many viable nameplate addons available. The most common one is Plater which is likely the best as it has unmatched customisation options, and like Grid2 etc. you can import other player profiles to give you a head start.
Method Raid Tools: This is an excellent addon that allows you to see cooldowns, notes etc. on your UI. Having a note provides the option of having your healer cds on your screen to be used within the fight, and you can also track other healer cooldowns as well.
An example UI can be something similar to this which is available on my twitch: https://www.twitch.tv/kalieyn
With Dragonflight, a change in has been implemented which allows healers to cast with mouseover without using a macro or addon. Please make sure you are doing this and not targeting a player each time to heal them, this is very important.
If you do not want to use the in-game setting, or you are having issues, use this and replace “spell/ability” which each one:
/use [@mouseover,help,nodead][help,nodead][@player] SPELL/ABILITY
#showtooltip Verdant Embrace(Green)
/use [@mouseover,help,nodead][help,nodead][@player] Verdant Embrace(Green)
WeakAuras are a must in your UI. WeakAuras allow you to change and track basically everything you would ever need for any form of content. It is incredibly customisable, however there are hundreds of packs already made at your disposal. I would recommend browsing their website and trying out the ones you like, without cluttering your UI too heavily.
I would recommend tracking impactful buffs, defensives etc. Not only does WeakAuras help for your specific class, but also in general. Healers can track “Targeted Spells”, “Personal Defensives” etc on their raid frames through weak auras which can help with decision making mid combat etc. The options are endless.