so basically you're estimating as you go and your estimated tax payments will be variable based on that. I also saw a comment you are leaving payment for the cap gains to the end which could be an issue.
the main thing to be aware of is that the underpayment is assessed quarterly and by default the process assumes that your total income was earned evenly through the year so that 90% safe harbor amount is divided by 4 and the estimated payments to meet that amount should also be evenly paid each quarter - and note that estimated tax deadlines are not quite quarterly - Q2 is 2 months and Q4 is 4 months, but the amounts should still be a quarter.
So let's say you end up owing 20k taxes total for 2025. The safe harbor 90% is 18k. For no penalty you would need to pay 4500 every month and the remaining 2k balance with your return in April 2026. If you end up paying 4000, 4000, 5000, 5000 then you will have met the 'safe harbor' total amount for the year but will have an underpayment penalty for Q1+Q2 that cannot be fixed by an overpayment in Q3+Q4. However an overpayment in Q1+2 will roll forward and eliminate an underpayment in Q3+4.
If you have a significant tax event causing you to pay higher Q4 etc you can use Form 2210 Annualized Income method to specify the uneven income and payments to eliminate the penalty (and need to do same on state) but this can be a lot extra work and I would only suggest using this for some significant unplanned tax event like a Roth conversion or large cap gain. I ended up having to do it for 2024 due to a Roth conversion and plan to avoid in 2025. The AI method is not quite quarterly, it looks at your income thru 3/31, 5/31, 8/31 and 12/31 to line up with the ES dates.
If looking to avoid the AI method, from what you've described your method may be susceptible to underpayment in earlier quarters if it doesn't take into account your full income for the year (e.g. cap gains), and also beware in Q2 you will likely underpay if you just base it on May paycheck instead of June - you will be a month short on your Q2 ES (tho using 5/31 would be correct if planning to adopt the Annualized Income method).
Rather than paying variable ES as you go, you may want to take a stab at estimating your full year income and safe harbor amount based on some sort of average income for the RSUs and cap gains, err on over-estimating, and then make even estimated tax payments based on that. You can always adjust the estimated tax payments lower later in the year if your running estimate then comes in lower than you planned.
Not a CPA/Expert but hope this explanation helps think thru options and potential issues.
