The system process database from the EN15804 add-on by GreenDelta (based on ecoinvent 3.10) is online again on Nexus.
Nevertheless, I would always recommend the unit process database to see the supply chain, perform MCS, see and calculate data quality and so on, which is generally not possible with system process databases.
When using "Lazy/On-Demand" calculations, the unit process data sets can be also calculated very fast , within seconds, in the newest openLCA and a computer that is maybe a few years old or newer.