Merchandise store provides the most losing streak and relegation resistant source of income, but has a very long pay back period for the cash (over 7 seasons).
Arena upgrades, focusing on the areas where you're currently selling out and doing small incremental upgrades, generally have a 2-3 season payback period. But arena attendance (and the prices you can charge) is fairly dependent on your team's results.
You've got the safety net of a bot below you in the conference to prevent auto-relegation, and at least at the moment there are two teams in the other conference near the salary floor, so you might be able to trim a little salary and still win a relegation series if push comes to shove. If you're resource constrained for upgrading the team to compete for the playoffs, there may be some value in trimming the salaries and trying to play limbo with staying up.
The biggest long term economic benefit for your team is going to be a solid training plan and the right balance of staff & infrastructure to support that. And controlling staff salaries as a whole is pretty important -- for newer teams (or really most teams that aren't training farms for a national team) you probably should have a moderately low cost level 4 trainer and maybe a level 2 PR trainer (if you want the specialty, not really that necessary if you're not contending in a close league) and a level 2-4 doctor (massage specialty seems nice to me though the size of the effect isn't clear and BB-Justin posted on Discord recently that it doesn't actually allow your players to play more minutes, just reduces the impact of the GS drop when they play too many). Myself, I'm also running a sports psychologist since I'm trying to contend this season and have a potential U21 player -- but even then, I'm under $20k/wk staff salaries.