I feel I should emphasize that "upgrading" a property does not necessarily raise the value of the property by that same amount. You can pay a bunch for new counter tops or something that don't really add much value. They might make it rent faster or for slightly more money, but value seems to be driven more by location, sqft, and number of beds/baths. Now if that $10k in upgrades ends up with another bedroom or bathroom, then it might be a better financial move. It just depends what you're spending money on. Each property is unique.
I'd bet that it's not even going to break even from your Net worth perspective (Spending 10k might increase the value of the home by little or nothing in which case you spent money and saw little or no gain in net worth). Maintenance and repair is just a necessary expense that comes with property ownership. You absolutely need to budget for ongoing costs to a rental just like you would for a personal residence, so you'll need to have some plan for that in your FIRE budget. The 50% rule is designed to handle most of the common expenses related to supporting an investment property and making it mostly self sufficient, so if you're following the 50% rule, you're probably in good shape budget wise and may not need to do much tweaking to your personal FIRE budget.