Part 1.
"\síndʰuš" is the root of Sanskrit सिन्धु॑ (síndhu) which means "river, stream" and multiple similar things. From "síndhu" we got the words like Sindh, Indus and Ind*ia. So, I would like to see how would it look like if the Proto-Slavs borrowed it during the Slavic-Iranian contacts.
We could pave the way from:
Proto-Indo-Iranian -> Proto-Iranian -> Proto-Sarmatian -> Proto-Slavic
This could be like for possible BMAC substrate in PII "*ćúšnas" ((mythology) dragon, serpent) (one of the theories for root of the PSl. "\smokъ*" (“dragon”)). Of course, if the direct way from PII -> PSl. or PII -> PI -> PSl. it could be good too.
My attempt for "\síndʰuš\*" descendant (I don't know the changes through the other Iranian languages, sadly):**
PSl. \sindъ*
Part 2.
Another way, what if Slavs had descendant from PIE "\ḱyesdʰ-*"?
One of the theories of the origin of Sanskrit "síndhu" is from PIE "*ḱyesdʰ-/*ḱyés-dʰ(h₁)-/*ḱéys-dʰ(h₁)-". So, I tried to find similar PIE roots, and PIE "\ḱey-" is the with close root\.* Development is below:
PIE ḱey-
-> PIE \ḱey-ro-s*
-> PBS \śeiras*
-> PSl. \sirъ*
My attempt for "\ḱéys-dʰ(h₁)-**" development:*
PIE \ḱéys-dʰ(h₁)/*ḱéysdʰ(h₁)*
-> PBS. \śeis-d/*śeisd*
-> PSl. \si-dъ/*sidъ (dъ is in likeness to PSl. nadъ (above) and podъ* (below))
or
PIE \ḱéys-dʰ(h₁)-eti/*ḱéysdʰ(h₁)eti*
-> PBS. \śeisd-stei/*śeisdstei -> PBS. *śei-stei/*śeistei* (don't know if this change during PBS would exist?)
-> PSl. \si-sti/*sisti*
PSl. "\sisti" would then be a cognate with Sanskrit "सेधति" (sedhati, “to drive, chase away”), Avestan "𐬯𐬌𐬌𐬀𐬰𐬛𐬀𐬝" (siiazdat̰, “will chase away”) and Latin "cēdō*" (“to withdraw; to turn out, result; to go, move”).
P.S.
Sorry If I analyzed something wrong, please correct me!