In general, for the currencies, you have to look at the unique consumables you can buy with those currencies to get a baseline for what the conversion rate is on your server. I say consumables, because judging a currency based on low demand items is problematic. Results may differ between servers by the way.

For AP this is Dawn Prism. Also perhaps look at (tri-)potions. There are also fragments (I forget their exact name) you collect from Cyro dailies with which you can eventually buy (Akaviri etc) motifs or runeboxes containing emotes and stuff. Those additionally require a small sum of AP to purchase, so I would not blow all AP on, say, Dawn Prism. There's also the Golden Vendor. Buying a (DLC) monster helm/shoulder that you don't have with AP is still a decent way to spend, notably the Night Market shoulders that have been on sale recently (one still to go), which are otherwise impossible to get. That said, you can also spend gold at the Golden vendor. Dawn Prism has a demand, by the way, not because Triune is a good jewellery trait, but because there are writs that require it. An indirect way of converting AP is via those writs into writ vouchers, then into gold.

For Tel Var the main consumables are Hakeijo and alchemy parcels. In regard to the latter, I once ran some tests with the caveat that this was years ago. They, then, contained a decently high amount of Columbine. They probably still do. The total value came to 20% more than you could get via Hakeijo. Question is whether it's worth the hassle.

You'll get a similar uplift with anything that requires a bit of extra work. Like a 10% to 20% profit margin for making potions or glyphs. I personally never sell Hakeijo outright. I make gold quality armor runes. That said, I keep them mostly for myself, these days, to make builds with.

In regard to alchemy reagents, if they are valued below 40 gold on TTC and you can combine them into a poison with (dirt cheap) Alkahest, then you can sell the poisons to NPC vendors at a profit over what a player would pay. In other words, 2x ingredients + 1x Alkahest yields 16 poisons at an NPC value of 5 gold each. That's 80 gold for 16. The issue is, some players know this trick and will buy up cheap reagents, which puts a floor on the price of those reagents.

In regard to poisons: Enchants and status effects are currently so strong, I would be surprised if there was any player demand for those. I have tons of double DOT poisons in my bank that ZOS gave me as rewards and I never used. The only thing that springs to mind is Escapist poisons for PvP. It's just, if you use those as a PvPer, you're pretty deep into meta endgame and probably making them yourself.

Then in regard to potions, as an endgame player it's a similar story. The single most useful potion that people might, and I deliberately say "might", buy in volume is a tri-stat potion. Problem is, ZOS are giving away a good amount of those and they can also be bought with AP. You'd have to check whether the price of the ingredients now tracks those AP potions. What that means for profits, I'm not sure.

Spell power potions were once popular, but with the proliferation of crit buffs applying to both bars nowadays, and the equivalent AP potions, they're just not any more. I still use them on my healer, but I don't blow through them nearly as fast as tripots. This is reflected in the price of Corn Flower and esp. Lady's Smock being at historic lows. That said, you might sell Detection / Immovability / Magicka potions for PvP or you might sell invisibility potions, but I don't know how big the market is for those.

Circling back to Tel Var: I recently bought a bunch of random blue armor boxes to fill my stickerbook while selling the excess. This added up to a lot according to TTC. Even though the star item, a Powerful Assault ice staff, has way fallen in price since the curated boxes, weapons in a good trait may sell for 50K to 100K each on PC NA. I think when I priced it all up, even selling the excess was more profitable than flowers or Hakeijo, EXCEPT those items turned out to be VERY slow sellers. We're talking 1 to 2 months here. It's something for stickerbook completionists only. I sold a good few at the prices that TTC said, then sales dried up. Had to drastically cut prices on the remaining stuff. Sorry, I don't have a final analysis. I guess the point is, it's a little risky. Bearing in mind you get a curated box for ... what now ... I forget. That puts a limit on what IC items will sell for nowadays. The days of 1.7M for a PA ice staff are gone. You might still get someone who buys a weapon at a slightly elevated price, due to it having the right trait, but it's rare.