Well your phono box retailed for $1,000. in 2006.
The P5 with speed control added was about $1,650 retail.
Now the cart is $450 current $$.
So I would say the cart and phono box are below the level of the P5 already.
Mainly adding the Power supply box from Rega, and the subplatter. Would make the table even better than the cart and phono box.
The first thing to upgrade is the cart. (unless it is not very worn. I would use it up, myself, if I had it. 1,000 hours, then buy a better cart.) or keep it for a spare.
The phono box is good, but also not quite as good as the table.
Consider the TT with power and subplatter is $2,000 with a $800 to $1,000 cartridge is $3,000.
Your Phono box should be near it's equal.
$1,500 to $2,000 used and you GOT something.
I use a Bryston BP 1.5 ($2,600 plus the power supply it needs which i have anyway for the Bryston BP-26 $$)
For my P5. With a Benz Glider $1,000.
If you want a moving coil cart, then the best phono box you can afford is the ticket. Once you have a good one.. It is a keeper.
The electrical signal is super tiny from a cart. and getting it boosted well costs money.
So if it is keep the cart and phono box to make a lateral move with a new TT.. only aa little better. No Way!!
Go for the cartridge, and a good phono box.
If the cuurrent cart is pretty new. just get the phono box.
Replace the cart later when it is worn $1,000 hrs.
((I use a golf clicker to count the sides, then X3 20 min a side to calculate hours. Way better than guessing.)
I do not own an aftermarket subplatter. But it is something i want.
I have no plans to replace my P5.