Yes, having a large unconditional "standard tax credit" with no phase out would just be another name for a UBI.
Realistically what you have to do at that point is to use a flatter rate structure instead of a phase-out so that the credit is paid back by the time you get to higher income levels, but that's much better than the existing nonsense where the marginal rate structure goes all over the place from sometimes >90% at low income levels (from phase outs) to ~20% in the middle and then back up to 30+%.
You get a strongly progressive effective rate just from the credit, so at that point you can use something like a 35% fixed marginal rate and simplify everything else too. Interestingly this also means it doesn't even have to be an individual income tax. You could use VAT or charge payroll tax to employers and get the same result without individuals even having to file tax returns anymore.
Realistically what you have to do at that point is to use a flatter rate structure instead of a phase-out so that the credit is paid back by the time you get to higher income levels, but that's much better than the existing nonsense where the marginal rate structure goes all over the place from sometimes >90% at low income levels (from phase outs) to ~20% in the middle and then back up to 30+%.
You get a strongly progressive effective rate just from the credit, so at that point you can use something like a 35% fixed marginal rate and simplify everything else too. Interestingly this also means it doesn't even have to be an individual income tax. You could use VAT or charge payroll tax to employers and get the same result without individuals even having to file tax returns anymore.