To me it’s not as much a question of “security” as much as it is a question of “accessibility” at a book like Jazz/BetUS.
Been around 30 years / got millions invested in their brand - not gonna close up shop and stick people for 100k. To rebuild a book - advertising and payoffs to forums would cost a million easy. Cheaper to stick it out.
BUT the issue will be liquidity - both for books and processors. Book may have plenty - but when a processor has to limit transactions and keep them small to stay under the radar - if you need cash quickly - a book may not be able to deliver.
What’s appropriate to keep - to each their own - myself I say 1-2 days plays. What’s your max you might ever play a day - 3 games, 5 games, a dozen ? Keep that in there. Always easier to send in than take out. Manage as needed to withdraw or deposit to maintain balances. But don’t get caught up in “great” bonuses. Take what’s advantageous to you - but don’t go depositing 5k if you bet $400 a day - nothing worse than being illiquid.