I am pretty new to mushrooms. What would you call a "high" dose? So far I have tried, 2, 3 and 4g dried. I definitely could feel it physically and became emotionally very sensitive but I didn't feel anything spiritual.
The two truly profound spiritual experiences I've had were actually 22g "High Hawaiian" magic truffles (not mushrooms, but similar), and around 5g cubensis magic mushrooms, though I did eyeball it so it may have been more or less.
I would say that the best bet at consistency is to grind your whole batch to average the potency, then try a small dose to get a feel of your particular batch before proceeding.
One thing to be aware of is that you need a decent amount of everclear to perform the extraction, but once you've gotten everything out, you're going to want to reduce the alcohol by evaporating some of it so that you don't need to take several shots to get a good dose.
Also, boiling isn't a problem for mushrooms, psilocybin is heat tolerant to that level. Water, on the other hand, degrades psilocybin fairly quickly.
The result is properly disgusting (too sour and shrooms taste horribly and shrooms brought me almost to vomit), BUT - it makes the trips shorter (say 3 hours instead of more exhausting 6) and much more intense.
What OP describes with "really high doses", well, I went beyond with those 3g, not sure how much further. But there are few 'tricks' to make it happen. 1) be alone (so not good for those who are anxious and on first trip), so that nobody interferes with the experience and you can just go deeper and deeper if you want; 2) close your eyes, ideally laying on the couch/bed, having some relaxing soft music as a background (I had some shamanic sounds which were positively adding to the experience I believe). Visual sensory input and managing of the body is too strong for the brain to ignore, it will throttle the trip significantly.
After some time, losing gradually all my senses, losing any mental connection with all parts of the body, I ended up as a mist of atoms, egoless, just watching from distance, swirling in the music that was playing in the background although I didn't hear it directly (weird, I know, hard to describe). Imagine hot steam sauna, if you have light there you can see droplets swirling.
What was also impressive was coming down, since the process of re-discovery of my physical self, be it hand or breathing or eye muscles was more pronounced compared to losing it, one at a time. This part took easily 30-60 minutes if uninterrupted. And at the end the utter sense of peace with all universe. I am not spiritual/religious and this didn't change that, but I understood back then where religions come from and why they tell us more about ourselves rather than god itself (but that's my own conclusion). Profound, 1 in top 3 most intense experiences in life, repeatedly (didn't do it for last 10 years though, kind of took from it what I needed and could I guess).
Mind you, at any time, I could interrupt it and stand up, losing 90% of the current state. Maybe on higher dosages this wouldn't be possible so easily (but they say vitamin C or sugar terminates the trip pretty quickly, not sure never needed it).
Also, having issues, anxiety, going though harder parts of life ain't a good combination with any mind-altering drug, or any drug for that matter.
I'd suggest you try ~12g next; with knowledge it might be a bit too much but some comfort you know what itll be generally from the smaller doses. if thats not what you're after, then maybe steps at 18 and 24g.
Do pay attention to frequency warnings and such commonly cited; too much too often is a waste of good shroom. Also consider extracts, I personally found the larger doses hard on the stomach, even powdered, compressed, and rendered easy to swallow with honey. Vodka is a great solvent and can be evaporated back off.
I'm in Australia and 5g of dried mushrooms does not leave you pleasantly couchlocked, as you say. On 5g, you will have a massive, epic trip.
I have taken shrooms three times. My biggest dose was 3.5g. This was a transformative experience with ego dissolution and a thorough, extended series of interactions with my sub-consciousness and all of its complexity.
I think my biggest single does was about the equivalent of 4oz dried; great trip but the hangover was way too long.
https://www.erowid.org/plants/mushrooms/mushrooms_dose.shtml