Any advice on plans to DCA of approx. 1K per month in either VTI or SPY for retirement planning?

I would suggest you evaluate a systematic plan into a balanced portfolio - which has a good mix of Equity ETFs, Bond ETFs and Gold. Systematically adding to such a portfolio every month can help build a corpus over time. You can also run the algorithm we have made available at kristal.AI to see what is the suggested recommendation for your profile. Do note at kristal.AI we have made investments upto 50K USD totally free of any advisory fee or hidden charges. Do compare it to other Robos before you go ahead.

We also do not have any minimum requirement to start an account.

The funds that you have chosen are good choices for broad market exposure and for long term holdings. They are efficient and very cheap.

However note that these funds, VTI and SPY, are US domiciled ETFs. In other words, there will be a 30% dividend tax withholding. This means that for every one dollar of gross dividend that the fund pays out, you will only receive 70 cents.
You might want consider Irish domiciled ETF, as the dividend tax withholding is only 15%. I believe there is a S&P 500 ETF listed on the London Stock Exchange.

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https://investment-blueprint.com/