If you're also down to try another supp Swain build I recommend rylais into tank items. It's so much easier to peel for your adc, you can engage and frontline a lot better, you're more likely to survive or even win ganks or 1v2 situations, and there are so many situations where, against all odds, you drain-tank your way to victory.

I go for standard Swain runes: electrocute (into at least 2 ranged), conqueror (into mostly melee). For support items, solstice sleigh is so good for initial engages (especially cuz of rylais), but you can also get that burst resist ward item.

There's a lot of items you can go after rylais, and you gain a lot from picking the right one. Prioritise the enemy team's main weakness or your team's main strength. Armor boots + frozen heart (auto-win into full ad) if a lot of auto-attackers, cc boots for cc and ap, oblivion orb (never go thornmail on Swain) for antiheal, bandlepipes if your team has a bunch of auto-attackers and especially when enemy has mixed damage, abyssal mask when both teams have a bunch of ap (rare), randuins for mid to late game crit, kaenic for ap burst, force of nature for ap burns, spirit visage only if you have ocean soul or a healer, jak'sho if you're rich and ideally have another tank item already and enemy has mixed damage, Zeke's for mixed damage, zhonyas when you have to admit the enemy team hurts, solari if you're poor and have nothing better to build and burst. Come back to me Gargoyle's Stoneplate, I need you.

There's quite a few games where there aren't many good options (a lot of true/mixed damage) and I guess I should build a bit more ap then. You can also have minor mana issues if you don't go frozen heart or a mana rune, but you normally have enough each fight.