I agree that cloudwatch is dogturds, but want to dive deeper for illustrative purposes:

Your dynamodb solution isn't foolproof. It has throughput limited to the partition granularity -> in your case the lambda name. It's also relatively expensive and fairly slow to query in bulk (DDB is designed for OLTP).

I don't have direct experience here, but I expect slapping grafana on top of any disk basked source is likely to be cheaper, faster, and have better ergonomics. Once your logging is too much for a disk to handle (this will be later than you would've outgrown ddb, but before you would've outgrown cloudwatch) then you can bring something fancy in.