Transamerica being an insurance company, they should offer a stable value fund of some kind. Stable value funds are great places for cash, as long as they pay a decent rate and don't charge high fees. If you were nearing retirement, some money in a stable value fund to reduce volatility would be a reasonable way to go. My 457 plan offered a stable value plan that paid a hair over three percent until recently. I left some cash in that fund when I FIRE'd. Turned out to be a good move when the SHTF in 2008.
Cash in a retirement account when you are not nearing retirement will likely cause you to underperform over time, so that's something to consider in choosing the new investments. As I understand your options, you have a selection of funds, the target date portfolio express (which will likely be laden with high fee funds because, hey, we are an insurance company, and that's what we do), and the Schwab self-directed brokerage alternative. The existing self-directed brokerage alternative will map to Schwab, and the stable value fund you now hold will by default be invested into the portfolio express.
You have a month to research all the funds available to you and make decisions. I would start by looking at the expense ratios and performance of all the available funds and all the fees Transamerica will charge you. There may be a fee to use the self-directed brokerage option, and there will likely be a fee for the portfolio express, or at least a fund fee similar to a target date fund. Figure out which fund/brokerage allocation you want and submit the paperwork. Your self-directed account will map to Schwab automatically, and the stable value fund balance will be allocated per your selections.
Be sure to look for buried fees and expenses, because this is an insurance company. If any of the fund offerings are tied to annuities, I would pass on them. You can also evaluate a lot of common 401k plans through BrightScope. Look under the new parent company if they use the same plan provider with the same investment options.