Ok so first, I'm sorry to hear about this but really don't go crazy please, you're definitely not the first to have a IS38 blown sure and plenty of options down the road that will get you up n running with a much greater reliability long term.
Now, I have said it before, thing with these engines is everybody is supposedly able to tune them, and maybe they all run similar peak boost but the way the power is made and how all supporting parameters are adjusted is what makes all the difference both in end performance and reliability. Research, research, research. If you can't swear by a particular smaller tuner you're thinking about or don't know first hand the owners of the cars they mapped and how the have fared over time don't take your chances and go with a big name that at least in this platform and engine are all proven with thousands of cars around the world, especially at such off-the-shelf tuning levels of stage 1 and 2.
Don't cheap out on remaps, you are at worst talking about a mere couple hundred more for something which could have an impact of a multiple times higher cost.
That been said, yes the IS38 turbocharger is known for its shaft weakness, although as the revisions came along (there's been a few since 2014) cases were diminishing and the majority of cars post 2016 have been fine. Myself I've had two IS38 cars both mapped to 370-390bhp from two different tuners and both have been fine, plenty of others I know that are stage 2 and still fine, then I've read about stocks cars blown, it was really a lottery with chances dropping the younger the cars are... Due to this history, It's very hard / impossible to be able to categorically blame the tuner or the turbo unit, or partly both so it does not really make any sense to chase the causes of this any further as it won't get you anywhere (apart from making you mad probably). I remember in an earlier post of yours stating the tuner had trouble keeping the intake temps down which is rather unusual for a stage 1 map, more important though you also say you've had issues with him in the past which really begs the question why you took your chances with them. It's a rhetorical question, no need to answer it.
Where do you go from here: Are you happy with the car otherwise? Do you plan to keep it for some time?
If (and only if) the answer is no then head back to your original tuner, ask them to replace the turbo with a used IS38 from the used parts market (cost around 500-700EUR) and ask them to revert the software to the stock map (every decent tuner should be able to provide this for free, especially after such an incident) and move on.
However I don't think this is what you had in mind.
If you've answered yes then the good news are that instead of a costly replacement you can upgrade your own turbo to a much greater standard (uprated shaft, bearings, balancing etc.) and decide what power you want to run, you could simply leave it at stage 2 levels of 400bhp or go for more. For example, I've heard really good stuff about Dan (i think is his name) from Littco:
https://www.littco.co.uk/hybrid-turbos-1-c.asp
They offer lots of very affordable upgrade options depending on what you want to do and the fact you will be sending them your own turbo to serve as the base is keeping the cost down. FYI the housing itself has no issues and can run over 500bhp, it's the core/shaft that is prone to fail so it's an easy fix for a company that knows what they're doing.
For example the L380 MQB option with "Supercore" upgrade is just 660 GBP and you've got nothing to worry after that. I'd suggest calling them and explaining your situation and they should be able to advice you accordingly. Then after the turbo is sorted choose your next tuner wisely (a whole different discussion).
Yes it's not for free but much lower than other more serious failures and you will have removed a known weakness of the stock engine/turbo.
In any case I think it's clear you should not even consider an OEM turbo at full retail price (there is no cost covered by the warranty obviously as you said), as even if running stock power you could end up right in the same position after a while.
Plus there is no reason to do that really. The gen3 EA888 engine is one of the most solid that have come out from VAG in terms of reliability and tuning potential with the IS38 turbo being the only real possibility for failure which at least is easily addressed.
Let us know how you decide to proceed and remember **** happens all the time, as long as it's not something terminal don't beat yourself up too much. I don't know how power hungry you are but personally I'd have seen it as the perfect excuse to proceed with my hybrid turbo plans...