How difficult or easy is it to transfer from AGR program to Army Reserve TPU? Would you recommend me requesting via 4187 or ETS and reenlist?

Army Reserve AGR should be similar to ARNG, in that you need to request a voluntary REFRAD from AGR.
If your ETS date is coming up in the next 90-120 days and your intent is to still be in after AGR, just extend first.
The vol REFRAD should require a 4187 and likely an MFR from you. And some math, you will need to calculate how much leave you have remaining, plus how much leave you will accrue during that leave.
And you will have to complete SFL-TAP prior to leave.

There's always a danger when you ETS...my friend hit a snag going from NG to AR and lost 6 months....almost got DEEP SIXED...