1. If you deal with small amounts received from a source you could use also directly the cashu wallet, without any channel. But I would use that for small amounts, under 100k sats. From that cashu wallet you can do internal swaps anytime you want, into your Zeus onchain UTXO or any external UTXO if you like.
2. But if you want to consolidate multiple small amounts, let's say into a 1.5M sats will be better to use a LSP channel. You will have to consider also that sometimes swaps from LN to onchain of 1.5M sats could fail due to low liquidity. But in Zeus also you have the option to choose from multiple swap operators if one it fails.
3. Before you start receiving size correctly your channels.
   For example: you plan to receive 3 payments during a month of total of 1M sats. So open a 1.2M sats if you want to play around with a fix amount of 1M sats. Why? Because there are some reserves fees, commit fees and buffer reserve to take in consideration. So if you have a 1.2Msats channel filled up, send out only 1M sats. The remaining 200k you can send it later but also no max than 90% (because of the buffer reserve) and so on. You will never be able to send all at once.
4. No, once you send almost all your sats from that channel, IT WILL NOT REDUCE the size of the channel. The size of the channel always remain the same and YOU CAN REFILL IT AGAIN anytime you want. That's the whole idea of payment channels: spend and replace.
   Yes, could be a splicing option (not yet available for LND/Zeus) where you can resize the channel. But it doesn't make sense to have an automatic shrink of a channel once you send almost all sats from it. An expansion of a channel, yes, make more sense, but keep in mind that will have extra fees onchain, is practically like another channel opening.
5. You can have multiple channels, with same LSP or with different peers. When you send out, you can use MPP (multi-part payment) and could use multiple channels for a single payment.

I hope this explanation it clears your questions. If you have more questions, I am happy to respond.