I understand your concern, especially considering when light has miracle/sanctuary. However this has already been taken into consideration.
Firstly, it would not be far fetched to say that there are more efficient stall decks than a stall deck that is centered around this card.
For one, the upgraded card is attractive for earlier implementation to the field but has a higher upkeep cost,
Even if you were to use duo

(Seraph) or another duo, the upkeep on this card makes the use of PA on it fairly futile simply because
1: summoning creatures with an upkeep of 4

(upped) (5

whenever you transform back to Themis from Nemesis or more if having a

using creature on field like Seraph, Leafdragon, etc.) is more difficult than you think. For one, you need at least 4

pillars/pens. if you don't already have a

mark, and that is disregarding the need to actually place

creatures or

using creatures (Leaf dragon, seraph, etc) on the field and for another,
2: If you are worried about Miracle, well its fair to say it is impossible to use miracle AND keep Themis on field as Miracle absorbs all

left, so once you end your turn it is destroyed. Obviously you can replace with another Themis but then again how many are you going to have in your deck without causing card congestion?
3: Even if you only had a limited amount of

pillars and change to Nemesis to recover

in order to use it for creature summoning
or Stalling or such, The opponent has that turn or more to deal creature damage and on top of that, once you change back, you are going to use
5

on that same turn due to the cost transforming back to Themis.
That upkeep is actually really heavy on a Themis focused stall deck and its efficiency is fairly questionable when compared to other more efficient
(not to mention upkeep-free stall decks) if anything, this would actually deter players from placing this in a

deck out - stalling deck
4: You can't really justify a stall deck with Themis using a large amount of pillars to cover an upkeep as steep as 4

(5 per return to Themis) in a deck as it would again just cause congestion
5: Much less Rainbow decks can't really use Themis as a stall or even make any efficient use of it for that matter like a duo-trio involving

deck can,
due to its pseudo play cost of 8

and its upkeep of 4

(5 per return to Themis). (mayyyybe juust maaybe if they employ the use of FFQ or Fire flies &/or RoLs)
6: Just for the exclusive use on Themis, How many PAs are you going to have in your deck without causing card congestion? One needs to consider the probability of drawing both cards in hand earlier or when the time is right while trying to stall with other cards within the deck
+ Pillars/pendulums for the actual upkeep of Themis.
In conclusion, having PA just exclusively for Themis with the intention of making a stall deck around it, is honestly a waste of card space
unless you're going to use it on a card that is actually more worth using PC on (in the eyes of the opponent) like Sanctuary or Fire shield (if Duo

) or something else.
Scales of Themis was not designed with deckout/Stall decks in mind (Hence the steep upkeep cost), but rather its counter-part Scales of Nemesis as the main feature of the card which aims to punish your opponent for cc-ing your creatures especially if they initially had decent hp (whether mono or duo or more

)
Nemesis toggle back into Themis is to give you the option of sacrificing your attack turn in order to cc creatures without getting yourself hurt (despite being

and being able to heal back that damage fairly easily if you were to cc enemy creatures while in Nemesis)