I could basically sums up your experience as Safari is appalling at multi tab resource management. And it has been the case for 14+ years and counting.

It wasn't until Safari 18 before I have most of the rendering issues gone on sites I visit. Safari 26 is completely gone. I haven't encountered one since Safari 26.1.

With a lot of features done, I just hope Safari turn its attention to performance and snappiness of the browser. Multi Tabs doesn't work. For people who uses more than 30+ Tabs is when it start getting slow. Safari used to have an option to unload background tab and that usually fix 80% of the problem but it was taken out some years ago.