I've decided on the parts I want, mid drive kit from AFT Ebikes as well as the 25Ah battery pack from em3ev (6.8kg) however I'm struggling to find a suitable bike that will fit the battery pack.

I'd prefer a mountain bike so I can take it a bit off-road occasionally and new bikes I looked at (Apollo Xpert 29x, Merida big Nine 20-MD) don't have enough space in the triangle. Only 35-41cm along the top. Mainly seems to be due to the fact new bikes have that top bar sloping down at an angle now.
Any suggestions for a hardtail MTB with a large triangle?

See if you can find a used Fisher Sawyer 29er.