Bring your Ptrap up or your Sanitary Tee down.
AS noted by others you can cut the tail of the piece where it goes into the top of the Ptrap shorter ( thereby bringing the Ptrap up ), This will allow you to have the down hill slope you need for the disposal. You may need a longer tail piece from the bell on the basin into the tee from the disposal.
OR
I think this is the better option because your sanitary tee is a little too high.
You could cut your drain pipe shorter so the sanitary tee is lower.
Use a regular Ptrap ( non flexible, there is no need for the debris catching flex connection here ).
You also need to mount your air admittance valve properly. It is not high enough, put it as high up as you can in the space between the bowl and the back of the cabinet.
If you put it on a threaded connection just above the san tee you will be able to pull it out to change it if necessary.
The procedure described is accurate.
My advice:
Hire your plumber (or a different plumber, if yours doesn't have the equipment) to inspect the drain with a camera to confirm that the trap is the only part of the drain that is damaged. You don't want to dig up the trap only to find later that there are additional issues and you don't want to do "exploratory surgery".
If you wanted custom candlesticks you would not hire a butcher or baker, you would hire a candlestick maker. Have a tile professional give you a bid on the job, for re-tiling and repairing (or replacing) the shower pan and tile, explain the reason and have him/her include excavation and replacement of the trap and drain assembly in the bid.
Done correctly you can just replace the floor and lower wall tiles (the "pan"); most folks choose a contrasting or complementary color and tile style and it looks ok. Be advised that this involves some waterproofing steps and it is not a simple job. Tile pros are usually very competent with the plumbing associated with the shower; they know best how to set the trap and drain assembly to ensure that it is at the correct height for the finished tile surface. Have them include everything in the bid.
Alternatively, have your plumber give a bid on just repairing/replacing the trap with it already chipped out for him/her by the tile contractor. The plumber should work with the tile contractor to ensure the drain and stuff will be correct.
Note: prices for services are off topic here. Always hire through personal references, always hire reputable licensed contractors, always get multiple bids (it's ok to be honest and tell potential service providers that you are getting multiple bids).
P.S.- I live in So Cal and had my entire sewer main dug up and replaced (6' deep X 50' long) for $6,000...
Best Answer
Don't know what country you're in but from a physics perspective you could certainly do it. You seem to know the basics about the depth of the weir. The other things you need to consider is to avoid building an S trap. The tail from the P trap needs to come off of it horizontally with a 1/4" per foot slope to the drain and note the max lengths of pipe below. Sounds as if you've got it.