Just following up on myself.
I’m actually running my server with both intermediate certificates, the one pointing to ISRG ROOT X1 and the legacy one pointing to DST Root CA X3, in this order. This works for Firefox, Chrome, Edge, SSL Labs and even on Firefox on Android. However, Chrome on Android gives an error.
If I swap the two intermediate certificates, Android’s Chrome starts to like my server again, but then SSL Labs indicates a certificate ordering problem.
I know that I’m ahead of my time using LE Authority X1’s e045a5a9 intermediate, but I thought that I shouldn’t have to wait for ISRG ROOT X1 to be included in major browsers before sending the matching intermediate certificate. Or is Android’s Chrome just too smart for the new root CA? Perhaps Google implemented some kind of pinning for any LE-certified site which doesn’t expect and accept an alternative certificate path to other root CA beside the legacy DST Root CA X3?