What would the steps be to implement a L1-less rollup node (sequencer)?

[ntn-x2]

If I want to build a EVM-compatible rollup that does not rely on the L1 (so technically a validium), what are the steps I would need to take to make it reth work for my use case?
I want the rollup node to:

1. Be compatible with Metamask out of the box
2. Collect transactions coming from users
3. Validate those transactions using the provided EVM engine (sorry I am still not very familiar with the terminology here)
4. Every N minutes, batch those transactions into something that looks like a block (but it's a batch, really)
5. Do something with this new batch of transactions

Is this currently possible, one way or another, with what reth currently provides? How much effort would that be?

Thanks all that answer my question!