This is exactly my dilemma. I am trying to avoid spending a whole lot on upgrades, in that I do suspect I'll want to trade up in the next couple of years.
I get your point about upgrades. In your position I'd upgrade items that make your sailing and living on the boat more comfortable/easier/enjoyable now
. It's your home. Anything can happen in a few years and you may decide on a different path altogether. Upgrades won't increase the value of the boat when you decide to sell, but as a buyer, it says to me that you took care of the boat and didn't neglect it. I'd choose that boat over one with absolutely no improvements since it was built.
For example, larger boat equals higher cost.
Primarily due to more equipment to maintain (i.e., inboard motor, head, etc). While my boat is (obviously) not maintenance free, it is extremely low maintenance. If my outboard motor goes, I replace that for a couple thousand, whereas if I had to replace an inboard, that would run in the ten to twenty thousand range. Moreover, besides higher bottom cleaning costs, there's cost of replacing zincs, packing the shaft, keeping the bilge as "oil and fume free" as possible as well as additional safety measures such as CO2 detectors, etc. Then, there's the head and dealing with pump-outs. Which would mean motoring over to the pump out station or hiring someone to pump her out at her slip.
Learn how to DIY. Diesel repair and head repair are probably not beyond your capabilities (if I am reading you correctly from hanging out in the chat). That'll save you money. If you list all of the maintenance items that a boat requires, probably 90% can be DIY if you lean that way.
As for pump out, at least in the Chesapeake the cost is minimal ($5/per, usually free if you fill up with fuel at the same time) and unless you're regularly creating enough poop to sink the boat, it'll barely be a smudge on your budget spreadsheet.