Keep in mind this answer is going to be based on personal experience, since that is what the question seems to ask for. Each of my yearlong campaigns in the last four years have progressed beyond 20th level, and I pass out XP granularly after every combat and event.
Number of Boons
Personally, I have found that if only one or two boons are acquired, each class has a couple fairly obvious complements to the skillset. Full casters will want High Magic, rogues Undetectability or Night Spirit, and so on. I'm assuming the kind of situation you're talking about includes something like half a dozen to a dozen boons; more and everyone just has the same checklist to go through.
Balancing Combats and XP
A "Deadly" encounter for a 20th level character, as per DMG 82, is going to be 12,700 XP, and judging by how that number changes over the previous few levels, should rise by 2,000 XP per level beyond 20th. Now, most of the boons aren't as powerful as an increase in level is (at 17th, a full caster gets a 9th level slot AND a cantrip damage boost AND additional health), so I personally estimate each boon as adding 1,600 XP to the "Deadly" level and follow a parallel logic at lower levels of challenge.
If a character is handling 6 or so of these per adventuring day, they will "level" every adventuring day, usually getting twice the 30,000 suggested value, but any given solo is not likely to do it - the Tarrasque itself yields a boon only if taken down by 5 or fewer adventurers.
You're right in that at these levels, ancient dragons and the Tarrasque become solos worthy of the party, and to keep some more complexity in the fights instead of just increasing enemy HP and damage, I include more than one enemy in most combats. A mating pair of dragons, or a pit fiend and a half dozen of his generals would provide a worthy challenge. The "Encounter Multipliers" table on DMG page 82 (used for balancing this) creates what I call "phantom XP" - XP used for calclating difficulty but not, RAW, given to the players. If you, as I do, give them this phantom XP anyways, it simplifies the progression, but means they will gain XP at a rapid clip of about 70,000 per adventuring day.
Given that a player character usually faces 7-9 "Medium" encounters or 4-5 "Deadly" encounters to level (DMG page 82 and PHB page 15), that comes out to a bit over one adventuring day per level. As I previously mentioned, most of the boons are a bit less of a power jump than a level, and less complex certainly than a new class feature. However, I don't want to pass them out twice a day, or after every third combat. Certainly more often than once a day feels contrived. Therefore, I personally set the threshold at 55,000 XP per boon. This measures out to once every 4-5 combats, which feels in line with the progression of previous levels. I also instruct my players to alternate ASIs with named boons to maintain a variety of bonuses; it also helps to regulate the difference in value between the different boons, since ASIs affect all classes equally, but YMMV.
Magic Items and Balance
It's fair to start by saying that DnD is not as well balanced at high levels (15+) as it is at low ones, to say nothing of beyond 20. It is for this reason that published adventures rarely go beyond 12-14, and Wizards has published virtually nothing for 20th level characters. Balance at this tier with magic items is going to be constrained by balance at this tier at all, which is to say, not very standard across parties. That said, my players can generally acquire any non-artifact magic item they want past 20th, though I keep a very close eye on anything with a +2 bonus or better and have added attunement requirements for +3 to keep bounded accuracy relevant. My players also understand they can't stockpile something like wands of fireball and drop a 9th level fireball each turn.
I feel that the epic boon progression is indeed too fast; 50,000 XP to 20th level should not be followed by 60,000 XP to two additional 9th level slots (High Magic and Spell Recall). I nearly halve the progression to 55,000/boon and require ASIs to be taken to further slow the progression and aviod blowing through all the relevant boons in 15 combats.