do you think there are enough I2P peers for a reliable Bitcoin node?
I don't know whether there's any reliable data about total Bitcoin node that use I2P.
https://bitnodes.io/nodes/ currently shows about 22.8K reachable node. But if you search ".i2p" on the search column, it shows 0 node (which should be incorrect).
Would using I2P affect transaction propagation or block syncing speed?
Yes, it's definitely slower than using clearnet. Just like Tor, I2P use multiple hops/nodes to perform communication.
https://geti2p.net/en/about/performance explain about it in detail.