no software is fully updated for 2021 since the tax laws can change substantially between now and then. so virtually any app you used will be based on 2020 tax law.
online isn't really practical for multiple scenarios.
Turbotax has taxcaster
https://turbotax.intuit.com/tax-tools/calculators/taxcaster?priorityCode=6099000241&PID=2278782&cid=...
it's not that sophisticated but can provide a reasonable estimate depending on your situation again more of 2020 law so the reasonableness of the calculations depends on your situation.
perhaps better is the 2020 desktop software. once you buy it (full pc windows 10 or mac OS?)
you can do as many scenarios as you want without any additional cost. however, you must be aware it's using 2020 tax laws some of which will not be in effect in 2021.
The desktop does under the estimate section have the ability to allow entry of 2021 estimated numbers to calculate your taxes. this column does not use any of the numbers enter for preparing 2020 and like other means is at best an estimate.
