How to set up LndHub on top of RaspiBolt ir RaspiBlitz? My goal is to support multiple LN clients without having to run a full node for each client.

How to set up LndHub on top of RaspiBolt or RaspiBlitz?
The RaspiBlitz is running LND. So, when you look at https://github.com/BlueWallet/LndHub, you will find that you only need to install Redis and then you can set up LndHub.io as in their documentation:

git clone git@github.com:BlueWallet/LndHub.git
cd LndHub
npm i

Install bitcoind, lnd and redis. Edit config.js and set it up correctly. Copy admin.macaroon and tls.cert in root folder of LndHub.

You can do all of that via ssh on your RaspiBlitz.