What are the differences between SRS operators, if any?
Not much difference. Fees and charges are all waived by DBS (and I believe OCBC too, but they do not state so online). UOB also waives all fees and charges except for Gold Certificate, Inter-Bank Transfer of account and Cancellation of Trade. All three operators give the standard interest of 0.05% pa for balances.
I suggest picking the one that is most convenient, like the one you do most of your business with.
Once I pick the SRS operator, what products can I invest outside that operator? Fund? Bond? ETF? Say, can I still buy ETF at SC's 0.18% instead of 0.275% at the SRS operator?
To buy exchange-listed stocks, bonds and ETFs, you can just tell your regular broker your SRS operator and account number and they will link it up. Unfortunately I think SC online trading does not allow SRS funds.
Besides exchanges, each SRS operator offers its own fixed deposits, bonds, unit trusts, insurance and other products but I am not sure whether regular non-HNWI investors can buy, say, HDB bonds.
Another way to access unit trusts is to use platforms like Dollardex or Fundsupermart that accept SRS funds.
Dollardex says that I can only invest the SRS contribution the year after. Is that true for all investment or is it DD only?
Where did you see this? I think you should be able to invest it soon after you bank in the money, not the year after.
What products do you recommend to put in SRS versus CPF versus cash accounts?
Tax implications: Not much difference. Dividends and distributions that go into SRS and CPF accounts are not taxed, but then again REIT distributions are not taxed at all and many company dividends are not taxed at the individual level. Capital gains are not taxed for any account. Lastly, even if tax is applicable, marginal rates are quite low so the tax shouldn't be too painful.
Investment restrictions: CPFIS and to a lesser extent SRS limit some investments such as stocks and funds, so naturally you can use your cash account for those if you have to own them.
Liquidity and time horizon: As SRS funds cannot be withdrawn until age 62 and CPF funds (above the MS) age 55, logically we would want to have long-term investments in those accounts, not e.g. fixed deposits. However, if you are already approaching either age, that doesn't apply. As CPFIS imposes custody and transaction fees based on counter and activity, it does not pay to trade actively using your CPF money. "Play" with your cash account if you must.
Lastly, a minor issue. For now, buying and holding stocks with CPF and SRS accounts means you cannot attend the company AGMs in your own name and you cannot vote your shares. They are changing the law but it will still take a few years. This is only if you want to attend AGMs and ask questions and vote.
