There is no what if feature with online turbotax . Using the download/desktop versions, you can do multiple trial returns.
I usually refer people to this tool https://turbotax.intuit.com/tax-tools/calculators/taxcaster/?s=1 But, I don't think it can handle a SS lump sum.
SS Lump sums, for past years, get special tax treatment, from the IRS. You do not just add the $70K to your 2019 return and pay tax at marginal rates. Essentially, you calculate tax "as if" you had received it in the year(s) it is designated for. TurboTax can handle that calculation. You need to have copies of your past returns handy.
You do NOT amend past returns. The special tax treatment effectively calculates tax "as if" you amended.
