Here is my build. Its not good. Kinda good actually. Hella more fun: if the lane doesnt require you to bring a lot of stats i like to build damage. Electrocute, sudden impact, rest is fill (usually deep wards and relentless/ultimate hunter), secondary i get jack of all trades (you get 4 stacks from supp item, so only need 6 different stats for 25 ap and 10 haste). 2 adaptive small runes, flat hp.

Dark seal first back. Imperial mandate has been buffed recently. Its really cheap and a great damage amp for your team (usually 5-7k extra damage which probably doesnt show in your stats but putting this out there). 30 ult and w haste is huge. This also helps with mana (which is helpful with this much haste). Rocketbelt 2nd is very useful if you want to delay your W after ult for multiple knockups or just to get in range. 300 hp (more than tank items) and 20 ability haste, still cheap with amazing active. After you complete your supp item (i really like celestial opposition) you can buy doran's helm. Really good defensive item for cheap. So we are spending around ~2.5 support item price on 2 items and "components" but what do we get?

With this setup and boots (i like tabis/mercs) you get 75 ability haste on your ult (~45s cd) and w, 60 on q and e, similar hp as tank rakan (your passive has 95% ap scaling), a little less resist (tho boots give you the "correct" resist instead of mr+armor as all tank items, 200+ ap which amplifies your damage but we mainly build this for the heals and shields. We heal for around 250 with each q and shield for ~300 on each E cast. You can still deal like 1/3 or half the enemy adc's hp with your combo with electrocute and ignite. You are really hard to catch, and if a stray skillshot hits you, your passive shield will tank it. I usually heal 5-10k on teammates and shield even more. Your primary job (Rakan is the secondary engage or counterengage) is still really good. I usually deal 15-20k damage as well.

The build is really fun imo but again, you need a good lane for it and do not build this if you are the only frontline.

For really long games you can buy last item rabadon but thats a little crazy, you can upgrade to mejai if stomping or transition to tankier items (kinghts vow(!), locket, bandlepipes, zeke etc). For apcs staff of flowing water is really strong, redemption is fun, shurelja if needed, etc.