how can I get the RICs for a list of tickers, so that I can work with get_timeseries?
I have seen this related post: https://community.developers.refinitiv.com/questions/19371/converting-a-list-of-tickers-to-rics.html
What is the best practice today to find out the RICs?
The thread you referenced is quite old and the answer it provides is outdated. I would recommend using the lookup capability in RDP Library. See examples here and here. Note that stock tickers are not globally unique, so to unambiguously resolve a stock ticker and convert it to a RIC you need to provide additional criteria, for example exchange or exchange country. For German stocks you could use
rdp.lookup(view=rdp.SearchViews.EquityQuotes, scope='TickerSymbol', terms='DAI,LHA,DBK', filter="ExchangeCountry eq 'DEU'", select='RIC')