Make sure none of the exceptions are ticked and the Minimum number of articles to keep per feed is also 25 or below. Then its up to the cron when that runs so you might have to manually purge it and optimise the database to see what it will actually keep.
I can’t say I have ever worried about it, been running FreshRSS for years and it seems to keep its database size in check fairly well and the defaults have worked fine for me and it rarely gets above 100MB. So I know it “loosely” works in that old articles are absolutely getting purged in time but have no idea how strictly it follows these rules.
I use a 5600g on b450 ITX board and 4x 8GB Seagate drives and see about 35W idle and about 40W average. It used to be 45W because I was forced to use a GPU in addition to a 3600 to boot (even though its headless, just a bad bios setup that I can’t fix) and getting a CPU with graphics dropped my idle consumption quite a bit. I suspect the extra wattage for your machine is probably the bigger motherboard and the less efficient CPU.
It is possible to get the machine part down into single digits wattage and then about 5W a drive is the floor without spinning them down, so the minimum you could likely see with a much less powerful CPU is about 30-35W.