There is nothing about referendums in the US constitution, and you cannot change the constitution by a referendum. The constitution can be changed only if 2/3 of congress (or 2/3 of states) ask for a new amendment and then 3/4 of states ratify it. That is how it was amended 27 times.
Having a bill for abortion in the constitution essentially make abortion as bullet-proof as it can be, but doing that is impossible considering that 26/50 states actually want to criminalize abortion. The next best thing (if Roe vs Wade gets overturned) is for the congress to pass a federal law that legalizes abortion. However, that has several problems: a) doing such a law requires the Democrats to break the filibuster which is hard to be done with Manchin and Sinema and potentially extremely dangerous for the future (as we saw with Dems breaking the filibuster for judicial appointments), b) states might complain that this is states territory, not federal government, complain in supreme court who might side with states, c) GOP might overturn it the next time around, and considering the precedent, criminalize the abortion in federal level.
Roe vs Wade gave the highest protection cause it essentially was a federal law that states couldn't do anything against it. Until the Supreme Court changed, in which case, the abortion decision goes back to the state level.
Feel like the Dems do not have a clear path to do anything here. Even if somehow Thomas dies, I am extremely skeptical that Roberts would vote to overturn a decision made by his own court. The only reason why Roberts does not vote for overturning Roe vs Wade is cause it is an established precedent, not because he thinks that the decision is either legally or morally correct.