bitcoind has finished its sync, now I can start electrs. I wasn't sure how slow the sync would be so I tried it first from scratch but it was going very slow, would probably take days, so instead I copied over the data from my old disk, which was around 50 GB.
With bitcoind running, electrs running, I can start the mempool backend and my now local block explorer is working:
https://media.utxo.nl/wp-content/uploads/nostr/d/8/d8c3aee4a8967d6602e352d1522084b6e2b77d3f440b766413d27f564b87b235.webp