Isolated Margin vs Shared Margin on the Futures Exchange

Just working to understand Isolated Margin and Shared Margin.

Suppose I have $1000. To increase my purchasing power, I create a margin account of 20x. Because I did this, I start off with $20000 worth of buying power (I am thinking that a Margin account is similar to a Credit Card).

I then go on a type of "buying spree".

I buy $5000 worth of Etherium and $15000 worth of Bitcoin. Because of my 20x assignment on my Margin Account:

• if my Ethereum position goes down $250 (i.e. 5%), I loose it all.
• If my Bitcoin position goes down $750 (i.e. 5%), I lose it all.
• If Etherium is worth $1500 when I buy it ($5,000 of it), then if my
  $5000 position turns into a $4750 position, I will lose it all.
• If Bitcoin is worth $25,000 when I buy it ($15,000 worth of it), then
  if my $15,000 position turns into a $14,250 position, I will lose it
  all.

*by loose it all, I mean my "initial investment" ( in this my example, $250 for ETH and $750 for BTC).

Question:
Suppose Ethereum goes down 6% (lose money on ETH) – but – Bitcoin goes from $25,000 to $35,000 (win money with BTC)

My understanding is that with an Isolated Margin account, the loss will stay with Etherium. In other words, I would lose the $250 (i.e. 5%) – and that would be all.

But, of I had a Shared Margin Account, then the Margin Setting would automatically increase ( ex: 11x, 12x, 13x, etc.) to whatever is necessary in order to save the Ethereum. The program will liquidate (or partially liquidate) my Bitcoin position if necessary (creating a type of complicated mess).

Is my understanding correct?

Question:
In general, when setting up trading accounts, should one have

• -> An account for SPOT trading only
• -> An account for MARGIN trading (with 1x leverage) – my understanding is that people use these accounts to "park" funds that
  are not being used(?)
• -> An account for MARGIN trading (with higher leverage – ex: 10x, 20x, etc.)

Changing margin settings seems to be problematic when more than one position is opened (I know that there can only be one open/active position per asset).

Any help, hints or advice would be greatly appreciated.

TIA