Sorry for late reply.
1a) So I have to pay another 499 euros to upgrade from v4 to 2021, to be able to use Octane for C4D 2021.1.2 or newer which has support for the 3080ti? Using this link:
https://render.otoy.com/shop/upgrade.php
This would have been true in the past but OTOY is no longer offering perpetual licenses beyond v2020 so I don't recommend this route. v2020 is the latest perpetual license offered so it would upgrade to v2020 (which is of course older than v2021) so that's why I don't recommend it.
1b) And once the 1 year included maintenance has passed, I will still be able to use 2021 with a 3080ti, or will it revert back to v4 and I have to pay another 499 euros?
So after the 1 year passes, you would fall back to v2020 which can support the 3080ti.
2) If I have just the v4 license (which is apparently useless to me now that I've swapped out my GPU), and I pay 19.99 euros for one month of OctaneRender Studio Subscription. Will I be able to use the 3080ti with Octane for C4D (2021.1.2 or latest version) and be able to cancel the subscription after one month, or?
Correct, you can just cancel the subscription after one month, no issues there. You'd be able to use the latest version which supports 3080ti.
Kind of difficult justifying paying 499 euros for a "perpetual" license, if a year or two from now it doesn't support the latest GPUs. Especially when I mostly do 2D work these days, and only occasionally land something that requires Octane.
caffe3
This is a legitimate concern with other players like NVIDIA involved. For your use-case, monthly is probably best. It lets you use latest version which allows latest GPUs and you can cancel after 1 month or after 6 months.
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