Bump welcome.
Used to have a broom yellow Coupe. Not many yellow 'B's about.
Sounds like you have already tackled the major risk items.
If you don't do track work then I would question the worth of brake upgrades for road use. I/we have a 'B and both of us have over 6K miles of track experience (in other vehicles) but as you might gather we don't potter around in our 'B'. Neither of us have ever queried the braking ability of the 'B' on fast road usage.
Similarly the base handling is generally very good and our 2005 'B' has never let either of us down when seriously challenge on fast road use.
One aspect that 'B's can respond/feedback to is styling and looks. Your Long Life exhaust probably gives you the nice sound/feedback/looks. You could consider wheels and engine compartment detailing.
You really need to decide what you are looking to achieve, how it may affect your insurance policy/prices etc.
And of course there are the internal cabin mods / tarting up that can be done.
Another factor is do you like showing and getting nice comments, appreciation and engaging with fellow admirers or are you more interested the "power/performance/handling" boasting stakes (no offence meant but if you can't use it is a potential waste of money).
I've modified two of our past/current Fiats (Uno SX and Strada Abarth 130TC) with performance mods as we were heavily into track driving/sprinting. To be really honest some of the mods made were a waste of money and time. Others, like the suspension upgrade to the Uno SX were brilliant successes on the track and provided only marginal improvements for legal road use.
Finally "bang for the buck" is/must be a priority and admiring feedback. Both of these will give you continual pleasure of ownership without draining the wallet.
A personal opinion