Lost my Safari tabs while merging Safari windows

Given the period of time that has elapsed, yes, the time available to recover a closed window has sadly passed.

There are no Apps to recover the closed Safari window - as this data is just one element of transient/cached System Data that is will be automatically purged. When you close a Safari Window, this state-data is only retained for a short period of time - the exact period I’ve not yet had opportunity to establish through structured testing. A restart of the iPad will also purge this state-data from volatile system data.

For practical purposes, the opportunity to re-open a closed window is likely to be tens-of-minutes - not many hours - this being more than adequate to recover an accidentally closed window.

Closed Tabs are treated differently - the last 200 to be deliberately closed being accessible from a long-touch of the Safari “+” tab button.

You also mentioned that Safari actually crashed. If so, state-data may be lost anyway, as Safari had no opportunity to save the state of currently opened tabs. While you may not now be able to recover your lost tabs, you are hopefully now better armed with knowledge of methods to preserve open tabs and tab windows - and if the unexpected should occur, routes to recovery using built-in tools, or from a manually saved-state.

All may not necessarily be lost…

This article, authored by me for an earlier version of iPadOS, may provide some useful insight and pointers:

iPadOS - Safari - Lost Windows, Lost Tabs… - Apple Community

While there are some differences to iPadOS15/16 - of which I am yet to incorporate by way of an update to the article - the majority of the procedures remain consistent for iPadOS15.

Should you be not able to recover the lost tabs, with benefit of this information, will be better equipped knowledge and techniques to avoid recurrence.