The 15-minute adventuring day
You have encountered the 15-minute workday problem - Google it and be overwhelmed with advice.
How many encounters?
Your expectation that 1-2 random encounters will happen over "several days" is wrong. If you use the method detailed in the module and the Binomial Distribution the chance of a given number of encounters per day is:
0: 42%, 1: 42%, 2: 14%, 3: 2%, The mean number of encounters in a day is 0.75 but the median number is 1 (i.e. 50% of days will have this or less, 50% this or more).
A party that blows all their resources on the first encounter has a decent chance of being unpleasantly stretched.
Notwithstanding, based on the figures in the DMG (pp.82-84), an adventuring day should consist of 6-7 Medium encounters, so, even at the upper end you are not seriously going to stretch the PCs.
The above assumes a normal sort of distribution of encounters, say 50% Medium, 25% Easy, 25% Hard and Deadly saved for major milestone encounters.
However, this is a sandbox - some of the encounters will be beyond Deadly for low level parties.
Consider the last random encounter that starts with "G". Assuming that you roll an average number of enemies, this is a 2,000XP encounter (if you roll the maximum its 3,150): for 4 PCs, this is a more than Deadly encounter for levels 1-4, Medium for 5-8 and Easy after that. A low level party that encounters that and survives is going to feel like they've been put through the wringer. A high level party gets to feel awesome about themselves, particularly if they are the same party having the encounter a second time.
Add in the fact that due to the particular circumstances of the module the stakes of losing are much higher and every battle is a challenge.
Things look different on the other side of the screen
I have given up trying to judge which fights will seem like a challenge and which will seem to be a cakewalk to the players. As a DM you have access to all the information, you know how close the monsters are to dying and the players don't. Therefore, their perception of the risk can be wildly at variance from yours.
I have run battles where I have been sure we were heading for a TPK but talking to the players afterwards they felt they were in no danger and I have also been at the other end where what I felt was a trivial encounter they perceived as a desperate struggle for survival.
What are the encounters for?
You are assuming that the purpose of the encounters are to challenge the party in a life or death sense.
Consider that the purpose of the encounters may be to provide interactive scenery for the players. The encounters are very evocative and thematically linked to where the party is in the wilderness. Yes, the party may blow through them without breaking a sweat but it tells them what's going on here in a much more interesting way than death by DM narration.
Enough frame challenge: what's to be done?
- There is no reason why your rest cycle cannot be different for wilderness versus dungeon crawl. Outside in the tropical heat you just can't rest as well as when you have shelter and shade. Wilderness: Short Rest 8 hours, Long Rest: 1 full day, Dungeon: Short Rest 1 hour, Long Rest 8 hours.
- The 3 "slots" for random encounters feel artificial because they are artificial. If instead you determine the time to the next encounter using an Exponential Distribution (using \$\lambda =1.33\$ if you want the same average number per day) then your players will not know if the next encounter is days or minutes away. Any spreadsheet can do this for you.