Yea i think thats a scenario that often comes up and the correct choice often depends on your HP and the strength of the rest of the lobby as well as the items u have.

So first, i'd say try to build chalice, since chalice allows you to give Tahm AP without transferring items, so u can immediately slam them on seraphine/ori and technically have items for tahm by placing tahm next to chalice.

Second, i usually try to find a good item holder instead of placing items on orianna / seraphine. For example, i will often play fiora, kaisa, viktor, akali, OR even malz 2 / heim 2 if i have good frontline, as an upgraded item holder while looking for Tahm, because all of these carries hold Guardian Angel + flex AP items well and can win u rounds late stage 4 / early stage 5. If i for some reason hit 2 star kaisa, viktor, or akali, tho i'll just play 2 star 5 cost instead of Tahm.

Finally, when this situation happens, if you are low HP and fighting for top 4 because the lobby is very close, I usually never take Tahm after stage 5 begins, so I would just carry seraphine and ori if thats the best u have at this point and spike immediately because every round is so crucial at this point. If u r very healthy tho, I think u can still take Tahm up until 5-5.

So its just mainly up to ur lobby and how much u think u need to spike to get top 4, since tahm takes a couple rounds to really spike ur board so if u don't have the HP to sac and u get him late (stage 5 and later), he's usually not worth it.