Penumbra has a scheme for trustless 3rd party scanning that they use for private addresses that you might be interested in. It uses a "Fuzzy Massage Detection" cryptographic protocol that is analogous to a Bloom filter, so you can delegate the scanning to a full node and ask a stream of transactions, which include all the transactions you care about plus some other false positives. https://protocol.penumbra.zone/main/crypto/fmd.html