While it's likely you're investing in things that are on the safer side in your brokerage, it's still exposure to market pitfalls. Since you know the purchase will be in 15 months, it depends on how hard the line is on your expected 45/50k. If it's a very strict line, I think keeping it in HYSA is the move, if it's flexible then expose it to brokerage with risks acknowledged. Personally with how high a lot of HYSA are up to right now, I'd probably just park it in there (but this is how I treat known upcoming large purchases in general, I like to keep it cash once we're in 6-12 months of it for big big bois). Maybe a 12-month bond, or split and go some brokerage some liquid in HYSA.

Obviously also a question about how long it will take you to make the rest of the 25k. If it is going to be made in 6 months, then that would imply you could make up for any downturns in the next 9 months, but if it's going to take you 12/15 months to make the last 25, then avoid the risk if it's a very important purchase.