Given, there was no data collecting at the time I made the switch from BTC into ETH and back, then the only possibility to reconstruct the trace between the two blockchains would be ...
-the point of time and
-the amount of the coin (together with the coin market price)?
Right?
Maybe. Or maybe the wallets you were using leaked your IP address. Or maybe you later check those addresses through a blockchain explorer or SPV wallet and leak your IP address that way. Or maybe those addresses are used again in the future and leak information that way. There are a lot of other sources of information that blockchain analysis companies can draw on.
So if someone knows, that a particular BTC address is mine, and then I make some transfers to another BTC address of mine and only then make the switch to ETH, this person does not know, until which BTC address the BTC's are still under mine possession.
Maybe. Or maybe your addresses can all be linked together because they are all part of the same SPV wallet. Or you look them all up from the same browser fingerprint or the same IP address. Or even the software you are using includes unique identifiers within the transaction data itself.
If you are going to go through all this hassle just to avoid using a mixer (although I'm still not clear why), then why would you use anything other than Monero?