The Tascam DR-70D is a good recorder for this type of thing, because it actually has an adjustable output designed specifically to feed the mic input on cameras. So you won't need the attenuator cable. That 5-way splitter will cause a signal drop, but since you have some adjustment on the DR-70 it shouldn't be a problem.
What was the other Tascam recorder?
https://tascam.com/int/product/dr-70d/specCAMERA OUT connector
Connector 3.5mm(1/8")stereo mini jack
Output impedance 200 ohms
Standard output level -40dBV
Maximum output level -24dBV
The Sony FDR-AX53 has an adjustable mic input level, so between that and an adjustable output from the 70D you should be in good shape. You'd want the level on the camera input to be pretty low and the output from the 70D high since the preamp on the camera is likely pretty low quality.