CONVERT_IMPLICIT when inserting a Geography datatype into a Geography datatype column on Temp Table
I don't think this has anything to do with the spatial extensions. I think this is this problem here,

From that post, all these work without generating CONVERT_IMPLICIT

SELECT * FROM [Table] WHERE IsSomething = 'TRUE';

SELECT * FROM [Table] WHERE IsSomething = Cast(1 as bit);

DECLARE @TRUE bit;
SELECT @TRUE = 1;
SELECT * FROM [Table] WHERE IsSomething = @TRUE;


You can read about the reasons for this problem here.

If it is the spatial type you may try a CAST to VARBINARY(max).. CAST( geography::STPolyFromText('') AS VARBINARY(max))



If you can't afford to upgrade to a newer version of SQL Server, PostgreSQL with PostGIS is perhaps an option as it is free and open source and has the best GIS implementation.

CREATE TEMP TABLE xSpatialTable (
     id         int           PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY,
     GeogCol1   geography
);

INSERT INTO xSpatialTable (GeogCol1)
VALUES ( 'POLYGON((53.835134 -2.474670, 53.812436 -0.288391, 52.417944 -0.090637, 52.270286 -3.018494, 53.835134 -2.474670))' );