Do you think that a private pool like Foundry will collude with other mining pools for a 51% attack? And which centralized exchange will they attack?
If I'm honest I don' think so. However having such a large % of the network controlled by such a small number of pools fundamentally goes against the principles of a decentralized network.
If you do not make transactions during the attack, then your bitcoins are safe.
Safe from the attack, but (unless you act quickly) not safe from the inevitable market crash that would follow news of Bitcoin undergoing a 51% attack.