Defragment not working

You're running into something called the Command Memory Limit, which is documented here: https://learn.microsoft.com/en-gb/power-bi/enterprise/troubleshoot-xmla-endpoint#resource-governing-command-memory-limit-in-premium It's also something I will address in an upcoming blog post in a series I started here https://blog.crossjoin.co.uk/2024/04/28/power-bi-semantic-model-memory-errors-part-1-model-size/ (reading this post will provide some useful background).

If you're using an A2 with 5GB of RAM and your existing model is 800MB, then you have 5GB - (800MB plus some extra memory used by queries and sessions) left for your refresh, so say 4GB. The next thing you should do is run a Profiler trace against your model (see https://blog.crossjoin.co.uk/2020/03/02/connecting-sql-server-profiler-to-power-bi-premium/ for how to do this) and capture the Command End events when you run a refresh. From this you'll be able to see the peak memory used during the refresh as a whole and for just Power Query (see https://blog.crossjoin.co.uk/2023/04/30/measuring-memory-and-cpu-usage-in-power-bi-during-dataset-refresh/) and for just Power Query for individual table partitions (see https://blog.crossjoin.co.uk/2023/04/02/identifying-cpu-and-memory-intensive-power-query-queries-during-refresh-in-the-power-bi-service/). My guess is that the peak memory for the refresh as a whole is going over 4GB. If so, then you need to look at the Power Query memory usage number for individual partitions and see if there are any memory hungry Power Query queries - if so, they need to be tuned. If not then it's likely to be your calculated columns that are the problem still. Reducing the amount of parallelism during refresh may also help reduce the peak memory usage (see https://blog.crossjoin.co.uk/2022/10/31/speed-up-power-bi-dataset-refresh-performance-in-premium-or-ppu-by-changing-the-parallel-loading-of-tables-setting/). 

If peak memory for the refresh is a lot less than 4GB then it's possible that the problem isn't the refresh but that users are running memory-hungry queries during the refresh. Running a Profiler trace during a refresh and looking for Query Begin/End events will tell you if queries are being run; there is a new Profiler event coming very soon which will tell you more about query memory usage but before that happens it's impossible to know how much memory a query consumes in the Service but you should see it happen easily in Power BI Desktop by looking at what happens in Task Manager when you view or interact with a report. If this is the problem then you'll need to tune your model and measures to reduce memory usage. This antipattern is a very common example of DAX that can cause memory spikes: https://xxlbi.com/blog/power-bi-antipatterns-9/ 

If you are able to post some screenshots of your Profiler traces and the memory usage numbers please let me know - I'm curious to see what you find!

HTH,

Chris