Alight's home page mentions "Self Directed Brokerage Accounts (SDBA)", which in general means buying whatever you want provided it's an ETF or stock. There might be a way to still invest with low expense ratios.
I suspect your "dollar cost averaging" question relates to taxes. Money that moves around within a retirement account has no tax implications. It's only money leaving the account that could incur tax (and maybe penalties). If you have a pre-tax 401(k), you've paid no taxes on those assets. Everything distributed from the account gets taxed. If you have a Roth 401(k), taxes have been paid on the entire amount, so as long as you follow the IRS rules, you don't pay taxes.
The most likely outcome, despite how I started this post, is that Alight will force you to pick from a new selection of funds. Your goal should be the lowest expense ratio, because of two benefits: low costs mean more money for you, and low costs also tend to be passive index funds, which have a track record of beating actively managed funds. (SPIVA data shows the S&P 500 beats 80-90% of active funds, depending on the time frame).