Bob, your question has been discussed before. Signal and Encryption at rest and also Is deleting secure in its current form? are good places to start. See also, Disappearing messages - comparison - #15 by anon98261654 which links to some github threads.
Since you are using samsung note 9, than means you have android >v6 you have a hardware-keystore and SQLCipher database, as I understand it. You don’t have any linked devices, so you don’t have to worry as much about what happens to your burn-after-reading messages – there is only one place where you read them. We are also, for the sake of argument, assuming that all your contacts are always careful to delete messages on their end ASAP, keep all their devices sync’s, and so on.
I’m not sure I understand the distinction you are trying to make, between giving your device voluntarily to a forensics team, and having your device stolen/lost/confiscated. Are you implying there is a time-limit for how long they can try to unlock your device? Or that there is a tools-limit on what kind of things they might use? Or maybe that, you are handing it to a tier-one group, that has relatively low degree of training and tooling, and your device will not get to the tier-two-and-up groups that have increasingly more-sophisticated training and tooling?
In any case, I’m not sure the answer is well-defined. Almost all android devices have SSD-ish storage nowadays. Performing secure deletion on those is hard, or perhaps impossible. Are you using any kind of full-device-encryption tool, in addition to signalapp, with your android? Are you assuming that the forensics team is able to unlock your device, i.e. that they also have your unlock-passphrase? Are you assuming that you power-off the device before handing it over? If we are talking about a situation where they do not have your device-unlock, where you hand it over powered-off, and where you are using a strong unlock-phrase in addition to some kind of full-device-crypto, then that is a best-case scenario. And even in that scenario, I’m not positive that a sufficiently sophisticated forensics team with enough time and tooling could not breach security eventually, because deleting in a forensic-resistant fashion is hard. If you are giving them a running device, and your unlock-phrase, and have not utilized any drive-encryption besides that provided by signalapp, then I believe (but am not sure! this is a tricky area) that your messages might be retrievable by a reasonably-sophisticated forensics team, that was able to find and decrypt “deleted” areas of your storage.